Authors:Barak Hirshberg; R. Benny Gerber Abstract: Publication date: Available online 7 March 2017 Source:Advances in Quantum Chemistry Author(s): Barak Hirshberg, R. Benny Gerber Methods that can accurately describe the quantum dynamics of large molecular systems have many potential applications. Since numerical solution of the time-dependent Schrödinger equation is only possible for systems with very few atoms, approximate methods are essential. This paper describes the development of such methods for this challenging time-dependent many-body quantum mechanical problem. Specifically, we focus on the development of mean-field theories, to which Mark Ratner has contributed greatly over the years, such as the time-dependent self-consistent field method, mixed quantum–classical methods, and the classical separable potentials method. The advantages and limitations of the different variants of mean-field theories are highlighted. Recent developments, aimed at applying mean-field methods for large systems, and their applications are presented. Issues where further methodological advancement is desirable are discussed. Examining the tools available so far, and the recent progress, we conclude there are promising perspectives for future development of mean-field theories for quantum dynamics with applications to realistic systems in important chemical and physical processes.

Authors:Isidore Last; Joshua Jortner Abstract: Publication date: Available online 24 February 2017 Source:Advances in Quantum Chemistry Author(s): Isidore Last, Joshua Jortner Novel features of analysis and control of nanoplasma dynamics are manifested in elemental and molecular clusters irradiated by a near-infrared intense ultraintense laser pulse, where the laser energy pumped to the nanoplasma electrons is transferred to the cluster ions by Coulomb explosion (CE) and by electron–ion impact mechanisms. The contribution of the electron–ion impact was studied by a microscopic model, together with molecular dynamics simulations of the electron–ion kinetic energy transfer in the course of the electron–ion collision events. The simulations were performed for ionic (He+)N, (Ne+)N, and (Ne4+)N clusters containing weakly charged ions, as well as for (H+)N and (He2+)N clusters consisting of bare nuclei and electrons. The clusters were subjected to femtosecond (τ =30fs) laser pulses with peak intensities of I M =1015–1017 Wcm−2. The force F imp, generated by the electron impact kinetic energy transfer was found to decrease strongly with the exploding cluster radius R, i.e., F imp ∝ R − η , with η ~4–6. The electron impact energy transferred to the periphery ions of clusters (in the size domain of N =104–106) made up less than 2.5% of the maximal ion energy. The laser energy transfer to the nanoplasma involves the dominating contribution of the Coulomb energy and a minor contribution of the electron impact, with the cluster expansion and decay being governed by the CE mechanism.

Authors:Adam P. Ashwell; Mark A. Ratner; George C. Schatz Abstract: Publication date: Available online 15 February 2017 Source:Advances in Quantum Chemistry Author(s): Adam P. Ashwell, Mark A. Ratner, George C. Schatz We present a detailed study of the impact of ligand passivation on the electronic structures and optical properties of plasmonic Ag nanoclusters using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The clusters studied are A g 13 5 + , A g 25 S H 18 − , A g 25 N H 2 18 − , A g 32 14 + , and A g 44 S H 30 4 − . We find that the highest occupied ligand orbitals from S (3p) and N (2p) appear just above the conduction band, and this leads to significant ligand-to-metal charge transfer transitions at high energies. Dielectric screening associated with ligand passivation results in reduced HOMO–LUMO gaps and in an increased gap between the HOMO and the valence band associated with the Ag 4d orbitals. Ligand field effects result in splitting of plasmonic peaks, leading to reduced mixing between nearby single-particle excitations. The magnitude of these effects is found to decrease when thiolate ligands are replaced with amine ligands. We also find that, in the case of the A g 44 S H 30 4 − cluster, the ligands localize plasmonic excitations into the core of the cluster.

Authors:Arie Landau; Debarati Bhattacharya; Idan Haritan; Anael Ben-Asher; Nimrod Moiseyev Abstract: Publication date: Available online 7 December 2016 Source:Advances in Quantum Chemistry Author(s): Arie Landau, Debarati Bhattacharya, Idan Haritan, Anael Ben-Asher, Nimrod Moiseyev Situations in which a molecule in a given configuration is electronically bound while in another configuration is autoionized are widespread in nature. In these situations, the change in molecular configuration due to nuclear dynamics is the reason the molecule emits free electrons to the surrounding, ie, autoionizes. Such a situation may even happen to molecules in their ground electronic state, for example, it can happen to H 2 − : at some bond lengths, the molecule is autoionized, at some bond lengths its ground state is bound, and at sufficiently large internuclear distances a stable hydrogen atom and a stable negative charged hydrogen, H−, in their ground electronic states, are obtained. In addition, such situations can be seen in electronic scattering from molecules and in cold molecular collisions. For example, in a collision between electronically excited helium atom and a hydrogen molecule in its ground state, metastable complex He*–H2 is formed. As time passes this complex decays to helium in its ground state, H 2 + , and a free electron. In all these cases the molecular dynamics play a key role as the molecules are autoionized. This poses a problem, since the Born–Oppenheimer (BO) approximation is applicable only when the decay process due to ionization is ignored. Therefore, in order to study molecular dynamics and take autoionization into consideration, one should calculate the potential energy surfaces (PES) by imposing outgoing boundary conditions (OBCs) on the electronic wavefunctions. Doing so, the electronic molecular spectrum will be discrete (no continuum), where the PES will be either real (bound electronic states) or complex (metastable molecules that ionize). These complex potential energy surfaces (CPES) are what enables one to take into consideration the electronic autoionization in the molecular dynamics. Nevertheless, calculating CPES by standard quantum chemistry packages (SQCPs) is not a trivial task, since they were designed to calculate bound electronic excited states. Bound states lie on the real plane, unlike metastable states (resonances); therefore, explicit calculation of resonances requires modification of SQCPs. Several different possibilities for calculating CPES by modifying SQCPs are discussed in this review. Yet, the holy grail is to be able to use SQCPs, which are highly efficient codes, for calculating resonances without changing the codes. The main focus of this review will be on new methods, we have developed, that enable calculating CPESs from SQCPs, ie, without any modifications of standard codes. Such methods allow the calculations of polyatomic CPESs, as indicated by our preliminary results.

Authors:Vipin Srivastava; Suchitra Sampath Abstract: Publication date: Available online 5 October 2016 Source:Advances in Quantum Chemistry Author(s): Vipin Srivastava, Suchitra Sampath We present some initial results to show that Löwdin's two orthogonalization schemes, namely Symmetric and Canonical, can help us to understand certain important aspects of the brain's competence to learn and memorize. We propose that these orthogonalizations may constitute the physiological actions that the brain may perform to deal with certain types of memories.

Authors:Ingvar Lindgren Abstract: Publication date: Available online 12 September 2016 Source:Advances in Quantum Chemistry Author(s): Ingvar Lindgren A new form of time-dependent perturbation theory has been developed based upon the covariant-evolution operator (CEO), previously introduced by us. This has made it possible to combine time-dependent perturbations, like the quantum-electrodynamical (QED) perturbations, with time-independent interactions, like the Coulomb interaction (electron correlation) in a single perturbation expansion. For the first time quantum-electrodynamical perturbations can then be combined with electron correlation beyond second order. The experimental accuracy is in many cases so high that these effects have become significant. A numerical scheme has been developed where first-order QED effects are combined with the electron correlation and applied to highly charged helium-like ions. This scheme contains the dominating part of the higher-order QED effects and has been applied to highly charged helium-like ions, for which effects beyond second order (two-photon effects) have for the first time been evaluated. The calculations have been performed using Feynman as well as Coulomb gauge. In evaluating effects beyond second order involving radiative QED it was necessary to employ the Coulomb gauge.

Authors:Orlando Tapia Abstract: Publication date: Available online 30 August 2016 Source:Advances in Quantum Chemistry Author(s): Orlando Tapia The photonic scheme provides an abstract perspective to describing chemical and physical processes; it is well adapted for biologically sustained processes too. The scheme is used to help analyze semiclassic pictures in order for a deeper understanding of natural processes to arise. A q-state is not an object (eg, a molecule) but convoluted with a photon field, it hangs somehow on sensitive surfaces revealing an image constructed from q-events: these q-events are joint q-energy and angular momentum bridging probe-to-probing systems. Exchanges between physical states and probing ones establish a reality for a q-state. Thus, in the photonic scheme, a q-state may emerge as an image if appropriately recorded via q-events. Initially collected q-events seem to indicate a random process. However, after gathering these q-events in sufficient numbers, as in a two-slit example, a supportive image develops corresponding more and more to what is known as an interference pattern. Moreover, the unlocking of a spin-triplet state is used to illustrate applications: for instance, the opening required a path starting from a parent spin-singlet excited electronic state. A low-frequency multiphoton mechanism regulated by conservation laws permits the description of a triplet state activation. Of course, the materiality sustaining a q-state must transfer information that is richer than that a classical particle impact would convey. The use we make of quantum mechanics is basically the same that everyone does though without current interpretations; inclusion of photon fields makes the difference by providing quantum mechanisms to accomplish measurements.

Authors:Karen Abstract: Publication date: Available online 25 August 2016 Source:Advances in Quantum Chemistry Author(s): Dževad Belkić, Karen Belkić High-resolution quantitative signal analysis using the fast Padé transform (FPT) is applied to a specific problem (cerebral asphyxia) within magnetic resonance spectroscopy (MRS) for in vivo pediatric neurodiagnostics. Potential broader implications for the presented methodology are indicated for interdisciplinary research, including quantum chemistry. An iterative averaging procedure is introduced and validated, which could be automatically built-in, to provide denoised spectra, so vitally needed in clinical MRS. The full equivalence of nonparametrically and parametrically generated total shape spectra in the FPT is demonstrated. With subsequent parametric analysis, exceedingly dense component spectra are reliably reconstructed, both with the mixture of absorption and dispersion components (“usual” mode) and by setting the reconstructed phases to zero, in order to eliminate interference effects (“ersatz” mode). Via the ersatz components, the consequences and extent of the said interference effect are distinctly visualized for every overlap of closely located resonances or hidden resonances. Practical implementation of Padé-optimized MRS from in vivo encoded time signals in the clinical setting is hereby demonstrated.

Authors:Sven Larsson Abstract: Publication date: Available online 3 August 2016 Source:Advances in Quantum Chemistry Author(s): Sven Larsson Thirty years after the discovery of high temperature (HT) superconductivity (SC), no by all accepted theory exists. The Bardeen, Cooper, Schrieffer (BCS) model, hewed into the Bloch theory for metals, is unfit for local systems such as cuprates and organic superconductors. In this chapter, we will use a theory that dates back to Landau and Pekar, but we will avoid the effective mass approach by using a total free energy model, as designed for electron transfer problems by Marcus and Jortner. A diffusion equation is used to derive the resistivity in the local case. The original definition of Hubbard U by Mott as a metal-to-metal (or molecule-to-molecule) charge transfer energy will be updated by including the neglected negative terms. It will be shown that the absorption at 2eV in the cuprates is indeed due to Cu–Cu charge transfer, identical to the Hubbard U or Mott transition. The model accounts for bond-length fluctuations due to occupancy of d-orbitals (extended over the ligands), or in the molecular case the π orbitals, and this makes it necessary to make a distinction between adiabatic and vertical Hubbard U. U vert =1.5–3eV while U ad may be a few hundred times smaller. Organic SC in aromatic hydrocarbons will be shortly reviewed and found consistent with the general model. Finally, we will discuss SC in tungsten bronzes discovered in 1964 by Matthias.

Authors:Marcelo Hidalgo Cardenuto; Kaline Coutinho; Sylvio Canuto Abstract: Publication date: Available online 26 July 2016 Source:Advances in Quantum Chemistry Author(s): Marcelo Hidalgo Cardenuto, Kaline Coutinho, Sylvio Canuto Combining Statistical Mechanics and Quantum Chemistry it is possible to study solvent effects in spectroscopy and understand chemical reactivity in solution. However, once the thermodynamic condition can be incorporated, it is possible to advance in other important regions of the phase diagram. Hence supercritical fluids with temperature and pressure beyond the critical point can be studied. Supercritical fluids are of interest both for their remarkable physical chemical properties and the industrial interests. The critical point, however, is apparently not a thermodynamic condition amenable to quantum chemical calculations. This is because it is characterized by intense fluctuations and density inhomogeneity. The correlation length becomes infinite at the critical point. But for points close enough to the critical point the fluctuations disappear, and it is possible to get very close to this rather interesting point in the phase diagram. In this work we review some results for the spectroscopy of molecular systems in the supercritical region and the static dipole polarizability and the refractive index of Ar only 2K above the critical point. The refractive index presents some peculiarities, but it is well behaved as we pass at the critical point. The numerical value obtained of 1.083 is in very good agreement with the experimental value of 1.086. We contend that the proximity of the critical point is amenable to theoretical quantum mechanical studies possibly accessing new physical phenomena.

Authors:Lawrence J. Dunne; Erkki J. Brändas; Hazel Cox Abstract: Publication date: Available online 25 July 2016 Source:Advances in Quantum Chemistry Author(s): Lawrence J. Dunne, Erkki J. Brändas, Hazel Cox In this chapter we give a selective review of our work on the role of electron correlation in the theory of high-temperature superconductivity (HTSC). The question of how electronic repulsions might give rise to off-diagonal long-range order (ODLRO) in high-temperature superconductors is currently one of the key questions in the theory of condensed matter. This chapter argues that the key to understanding the occurrence of HTSC in cuprates is to be found in the Bohm–Pines Hamiltonian, modified to include a polarizable dielectric background. The approach uses reduced electronic density matrices and discusses how these can be used to understand whether ODLRO giving rise to superconductivity might arise from a Bohm–Pines-type potential which is comprised of a weak long-range attractive tail and a much stronger short-range repulsive Coulomb interaction. This allows time-reversed electron pairs to undergo a superconducting condensation on alternant cuprate lattices. Thus, a detailed summary is given of the arguments that such interacting electrons can cooperate to produce a superconducting state in which time-reversed pairs of electrons effectively avoid the repulsive hard-core of the interelectronic Coulomb interaction but reside on average in the attractive well of the effective potential. In a superconductor the plasma wave function becomes the longitudinal component of a massive photon by the Anderson–Higgs mechanism. The alternant cuprate lattice structure is the key to achieving HTSC in cuprates with d x 2 − y 2 symmetry condensate symmetry.

Authors:David A. Micha Abstract: Publication date: Available online 1 July 2016 Source:Advances in Quantum Chemistry Author(s): David A. Micha The subject of this contribution is how projection operators can be constructed to treat a variety of time-dependent phenomena involving interacting molecules, and to treat the dissipative dynamics of a localized subsystem in a large environment. It develops partitioning methods in a functional space of wavefunctions introduced to construct molecular effective potentials and long-lived states from distortion, adiabatic, and fast motion states. It also gives a treatment starting from the statistical density operator, for partitioning in a many-atom system undergoing dissipative dynamics, and shows how to construct contracted density operators with selected total system states, or reduced density operators for localized phenomena in a primary region. The presentation displays related mathematical procedures useful for partitioning of both wavefunctions and density operators and for derivation of their equations of motion.

Authors:Héctor H. Corzo; J.V. Ortiz Abstract: Publication date: Available online 1 July 2016 Source:Advances in Quantum Chemistry Author(s): Héctor H. Corzo, J.V. Ortiz Electron propagator theory is an efficient means to accurately calculating electron binding energies and associated Dyson orbitals that is systematically improvable and easily interpreted in terms of familiar concepts of valence theory. After a brief discussion of the physical meaning of the poles and residues of the electron propagator, the Dyson quasiparticle equation is derived. Practical approximations of the self-energy operator in common use are defined in terms of the elements of the Hermitian superoperator Hamiltonian matrix. Methods that retain select self-energy terms in all orders of the fluctuation potential include the two-particle-one-hole Tamm–Dancoff approximation, the renormalized third-order method, the third-order algebraic diagrammatic construction, and the renormalized, nondiagonal second-order approximation. Methods based on diagonal second-order and third-order elements of the self-energy matrix, such as the diagonal second-order, diagonal third-order, outer valence Green's function, partial third-order, and renormalized partial third-order approximations, provide efficient alternatives. Recent numerical tests on valence, vertical ionization energies of representative, small molecules, and a comparison of arithmetic and memory requirements provide guidance to users of electron propagator software. A survey of recent applications and extensions illustrates the versatility and interpretive power of electron propagator methodology.

Authors:Berrondo Abstract: Publication date: Available online 25 June 2016 Source:Advances in Quantum Chemistry Author(s): M. Berrondo, J. Récamier The energy transfer between vibrational and translational degrees of freedom during an inelastic collision between two molecules induces quantum squeezing in the vibrational molecular coordinate under very diverse circumstances. In this chapter, we present the relevant calculation for the very simple case of an atom–diatomic collinear transition, the Landau–Teller model. Both the uncertainty of the vibrational coordinate and the Husimi function show clear evidence of quantum squeezing. Our model treats the relative translation of the colliding species as a classical variable. The vibrational motion of the diatomic molecule is treated quantum mechanically in terms of the evolution operator and coherent states. The corresponding classical and quantum equations of motion are coupled. We first consider the squeezing of a coherent vibrational state where we include the dynamic evolution of the Husimi function. The second case corresponds to an initial thermal distribution of vibrational states where we plot the position uncertainty for the squeezed vibrational state.

Authors:Jan Linderberg; Yngve Erkki John Sabin Abstract: Publication date: Available online 14 June 2016 Source:Advances in Quantum Chemistry Author(s): Jan Linderberg, Yngve Öhrn, Erkki J. Brändas, John R. Sabin

Authors:Carlos F. Bunge Abstract: Publication date: Available online 8 June 2016 Source:Advances in Quantum Chemistry Author(s): Carlos F. Bunge Per-Olov Löwdin was an inspiring and compelling teacher. His most prominent papers were written more than 50 years ago, whereas since then quantum chemistry, its software, and its computers have changed almost beyond recognition. In accurate calculations with truncation energy errors, an important part of Per-Olov’s themes and thoughts appears highly relevant today in applications to atoms and small molecules. My purpose here is to place projection operators, natural orbitals, error bounds, and the variational theorem for finite Hermitian matrices, in the light of current challenges in the field.

Authors:Jan Linderberg Abstract: Publication date: Available online 2 June 2016 Source:Advances in Quantum Chemistry Author(s): Jan Linderberg Attempting to survey and review some developments in the design and use of atomic basis sets for molecular electronic structure calculations from the perspective of Per-Olov Löwdin's contributions this chapter is offered as a contribution to the celebration of the centennial of his birth.

Authors:Yngve Abstract: Publication date: Available online 31 May 2016 Source:Advances in Quantum Chemistry Author(s): Yngve Öhrn This account of the time-dependent variational principle is presented in memory of Per-Olov Löwdin on the occasion of the centenary of his birth. The material presented here has been published as part of a book chapter, 1 and is reintroduced here in recognition of Löwdin's interest in this topic. 2 Also the importance of the use of coherent state parameters as functions of time is emphasized, as well as the connection to the electron nuclear dynamics (END) theory. 3

Authors:Cleanthes A. Nicolaides Abstract: Publication date: Available online 30 April 2016 Source:Advances in Quantum Chemistry Author(s): Cleanthes A. Nicolaides I start by thanking Erkki Brändas and Jack Sabin for inviting me to contribute to this special volume commemorating the 100th birthday of Per-Olov Löwdin. Their initiative adds to previous ones involving conferences and books that have been dedicated to him, all expressing the respect and admiration that Löwdin inspired throughout his scientific career in his associates and professional colleagues. Although there are many people who are better qualified to comment on Löwdin's personality and achievements, I take this opportunity to state briefly my impressions of him. I met Löwdin only a few times during the 1970s and 1980s, in conferences and in Sanibel symposia, starting with the conference on “The Future of Quantum Chemistry” that was held in Dalseter, Norway, Sept. 1–5, 1976, organized by J.-L. Calais and O. Goscinski to celebrate his 60th birthday. Those encounters (which included a couple of cocktail parties and soccer games where he played goalie) led to a nice rapport, even though I was much younger. They were sufficient to leave me with the best of impressions about his openness, about his interest in assisting young scientists from all over the world, and about his scientific inquisitiveness and aim for mathematical clarity and justification. Löwdin's scientific and organizational achievements were instrumental in advancing the cause of quantum chemistry in Sweden as well as internationally, especially during the 1950s and 1960s. For example, the Uppsala summer schools and the Sanibel winter symposia became institutions. He is remembered with respect and affection not only for his research papers but also for his exceptional activity which accelerated the recognition of quantum chemistry as a distinct scientific discipline with a diverse community of theoretical scientists. As a member of this community, I feel lucky for the opportunity given to me by the Editors to contribute the paper which follows. The paper summarizes elements of theories and computational methods that we have constructed and applied over the years for the nonperturbative solution of many-electron problems (MEPs), in the absence or presence of strong external fields, concerning resonance/nonstationary states with a variety of electronic structures. Using brief arguments and comments, I explain how these MEPs are solvable in terms of practical time-independent or time-dependent methods, which are based on single- or multistate Hermitian or non-Hermitian formulations. The latter result from the complex eigenvalue Schrödinger equation (CESE) theory. The CESE has been derived, for field free as well as for field-induced resonances, by starting from Fano's 1961 discrete-continuum standing-wave superposition, and by imposing outgoing-wave boundary conditions on the resulting solution. Regularization is effected via the use of complex coordinates for the orbitals of the outgoing electron(s) in each channel. The Hamiltonian coordinates remain real. The computational framework emphasizes the use of appropriate forms of the trial wavefunctions and the choice of function spaces according to the state- and property-specific methodology, using either nonrelativistic or relativistic Hamiltonians. In most cases, the bound part of excited wavefunctions is obtained via state-specific “HF or MCHF plus selected parts of electron correlation” schemes. This approach was first introduced to the theory of multiply excited and inner-hole autoionizing states in 1972, and its feasibility was demonstrated even in cases of multiply excited negative-ion scattering resonances. For problems of states interacting with strong and/or ultrashort pulses, the many-electron time-dependent Schrödinger equation is solved via the state-specific expansion approach. Applications have produced a plethora of numerical data that either compare favorably with measurements or constitute testable predictions of properties of N-electron field-free and field-induced nonstationary states.

Authors:Carlos Mario Granados-Castro; Lorenzo Ugo Ancarani; Gustavo Gasaneo; Dario M. Mitnik Pages: 3 - 57 Abstract: Publication date: Available online 6 January 2016 Source:Advances in Quantum Chemistry Author(s): Carlos Mario Granados-Castro, Lorenzo Ugo Ancarani, Gustavo Gasaneo, Dario M. Mitnik An accurate theoretical description of photoionization processes is necessary in order to understand a wide variety of physical and chemical phenomena and allows one to test correlation effects of the target. Compared to the case of many-electron atoms several extra challenges occur for molecules. The scattering problem is generally multicenter and highly noncentral. The molecular orientation with respect to the polarization of the radiation field must also be taken into account. These features make the computational task much more cumbersome and expensive than for atomic targets. In order to calculate cross sections, one needs to describe the ejected electron with a continuum wavefunction with appropriate Coulomb asymptotic conditions. Making a number of initial approximations, many different theoretical/numerical methods have been proposed over the years. However, depending on the complexity of the molecule, agreement among them is not uniform and many features of the experimental data are not so well reproduced. This is illustrated through a number of examples. In order to have a global theoretical overview, we present a survey of most of the methods available in the literature, indicating their application to different molecules. Within a Born–Oppenheimer, one-center expansion and single active electron approximation, we then introduce a Sturmian approach to describe photoionization of molecular targets. The method is based on the use of generalized Sturmian functions for which correct boundary conditions can be chosen. This property makes the method computationally efficient, as illustrated with results for H2O, NH3, and CH4.

Authors:Telhat Ozdogan; Melek Eraslan Pages: 173 - 182 Abstract: Publication date: Available online 8 January 2016 Source:Advances in Quantum Chemistry Author(s): Telhat Ozdogan, Melek Eraslan An analytical formula have been presented for momentum density and Compton profiles of atoms using Hartree–Fock–Roothaan method. The obtained formula includes the linear combination coefficients of molecular orbitals, auxiliary functions B mn l (α, β; q) and K n l (α, q), and Gaunt coefficients. Computer programs have been constructed for atomic Compton profiles and including functions. Utilizing these programs, Compton profiles of atoms 2≤ Z ≤10 have been calculated for a wide range of incident photon energy. It is seen that the obtained results for Compton profiles of these atoms are in good agreement with the more recent theoretical and experimental works.

Authors:Alessandro Roggero; Paolo Mori; Abhishek Mukherjee; Francesco Pederiva Pages: 315 - 332 Abstract: Publication date: Available online 8 January 2016 Source:Advances in Quantum Chemistry Author(s): Alessandro Roggero, Paolo Mori, Abishek Mukherjee, Francesco Pederiva Quantum Monte Carlo algorithms in Fock space have gained popularity in the last few years. Here we review the Configuration Interaction Monte Carlo (CIMC) algorithm. CIMC provides a way to implement the imaginary time propagation projecting the ground state of a given Hamiltonian in a model Hilbert space that (1) makes use of an importance function, and in particular of the wave function computed in a Coupled Cluster calculation, and (2) exploits a continuous time algorithm to eliminate the approximations due to the use of a finite imaginary time step. Some results and discussions from the implementation in the three-dimensional electron gas and first row atoms are also presented.