1,721,022 research outputs found
Forcing broken symmetry to recover static electronic correlation: the H2 molecule test case
No full textA self-consistent field method based on a density matrix functional theory scheme is presented to compute the potential energy curve of the hydrogen molecule. A functional to recover the so-called cumulant correlation energy is introduced. The form of such functional is very simple being dependent explicitly only on the square of the overlap matrix element between the unrestricted non-orthogonal occupied orbitals of the calculation. The functional is a sum of two contributions referring to non-dynamic and dynamic correlation. The agreement with the full-CI potential energy curve is within chemical accuracy
Extraction of a one-particle reduced density matrix from a quantum monte carlo electronic density: A new tool for studying nondynamic correlation
Full text linkIn this work, we present a method to build a first order reduced density matrix (1-RDM) of a molecule from variational Quantum Monte Carlo (VMC) computations by means of a given correlated mapping wave function. Such a wave function is modeled on a Generalized Valence Bond plus Complete Active Space Configuration Interaction form and fits at best the density resulting from the Slater-Jastrow wave function of VMC. The accuracy of the method proposed has been proved by comparing the resulting kinetic energy with the corresponding VMC value. This 1-RDM is used to analyze the amount of correlation eventually captured in Kohn-Sham calculations performed in an unrestricted approach (UKS-DFT) and with different energy functionals. We performed test calculations on a selected set of molecules that show a significant multireference character. In this analysis, we compared both local and global indicators of nondynamic and dynamic correlation. Moreover, following the natural orbital decomposition of the 1-RDM, we also compared the effective temperatures of the corresponding Fermi-like distributions. Although there is a general agreement between UKS-DFT and VMC, we found the best match with the functional LC-BLYP
Many-body approaches at different scales: a tribute to Norman H. March on the occasion of his 90th birthday
No full textThis book presents a collection of invited research and review contributions on recent advances in (mainly) theoretical condensed matter physics, theoretical chemistry, and theoretical physics. The volume celebrates the 90th birthday of N.H. March (Emeritus Professor, Oxford University, UK), a prominent figure in all of these fields. Given the broad range of interests in the research activity of Professor March, who collaborated with a number of eminent scientists in physics and chemistry, the volume embraces quite diverse topics in physics and chemistry, at various dimensions and energy scales. One thread connecting all these topics is correlation in aggregated states of matter, ranging from nuclear physics to molecules, clusters, disordered condensed phases such as the liquid state, and solid state physics, and the various phase transitions, both structural and electronic, occurring therein. A final chapter leaps to an even larger scale of matter aggregation, namely the universe and gravitation. A further no less important common thread is methodological, with the application of theoretical physics and chemistry, particularly density functional theory and statistical field theory, to both nuclear and condensed matter
On the effect of solute-solvent Pauli repulsion on n → π* transition for acrolein in water solution
Full text linkIn this work, we present a method to compute the Pauli repulsion contribution to the solute-solvent interaction that exploits solute electronic configurations sampled by Quantum Monte Carlo simulations. Starting from the inspiring model of Amovilli and Mennucci, the discreteness of the solvent is recovered by the definition of molecular domains and the concept of probe molecule. The method can be calibrated on the solute ground state but it offers the advantage of being able to be applied also to electronic-excited states. We show the results for the formaldehyde-water intermolecular pair, here used for the calibration, and two clusters containing acrolein surrounded by 11 and 19 water molecules simulating the solvation shell. In these systems, hydrogen bonds are formed between the solute and the water molecules and we found that, in such case, the Pauli repulsion contribution gives a red shift in the n → π* vertical transition energy
Intermolecular Pauli repulsion: a QMC study of molecules in ground and excited state in free space and in solution
Full text linkIn this work we present a method to compute the Pauli repulsion interaction energy between two molecules and for a molecule solvated by a discrete medium. The method of Amovilli and Mennucci, that has been developed within a continuum solvent model approach, is here revised in order to treat the solvation environment with a discrete number of solvent molecules. In our model, one of the two interacting systems, and the solvent in the case of solvation, acts as ‘probe’. A probe has a volume domain defined by the atomic spheres centred on the nuclei of the relevant molecule. The probe measures the fraction of electrons of the solute molecule falling in its domain leading to the evaluation of Pauli repulsion energy. To this end, Quantum Monte Carlo calculations are used to sample the electronic configurations of the solute. The method has been designed to be applied also to excited states. We show results for test systems in the ground state and for the ground and the (Formula presented.) excited states of acetone in a cluster with 14 water molecules
Calculation of potential energy surfaces with quantum Monte Carlo as a useful tool for the design of green chemical syntheses: The HOCO radical test case.
No full textPoster P15: Excited states of molecular solutes with Quantum Monte Carlo: vertical transition and geometry optimization
No full textRecently, we have developed a novel Polarizable Continuum Model (PCM), which includes both surface and volume polarization of the dielectric medium (pure SVPE scheme), designed for the Quantum Monte Carlo (QMC) treatment of the solute. In particular, the treatment of volume polarization, due to quantum mechanical penetration of the solute charge density in the solvent domain, is based on quantum Monte Carlo techniques. The method allows to accurately solve Poisson's equation of the solvation model coupled with the Schrödinger equation for the solute [1,2,3]. The present model has been now extended to treat the effects of solvation in solute vertical electronic transitions and to the search of the solute equilibrium geometry in the excited states. For the first case, here we show the results of our study
performed on fast n → pi* and pi → pi* vertical transitions of s-trans- acrolein in water [4]. To perform calculations in a non-equilibrium solvation regime for the solute excited state, we have added a correction to the global dielectric polarization charge density, obtained self consistently with the solute ground-state wave function by assuming a linear-response scheme. The calculated solvatochromic shifts are properly described. For the second case, we start from recent
advances made to carry out the ground- and excited-state geometry optimization within QMC [5]. For the present purpose, we have extended the calculation of the forces to include solvent e_ects through our QMC implementation of PCM [6]. We show results, performed at the variational Monte Carlo level, on the excited-state geometry optimization of some small organic molecules in water solution and we make a comparison with the more widely used TDDFT and CASPT2 methods.
[1] C. Amovilli, C. Filippi, F. M. Floris, J. Phys. Chem. B (2006) 110 26225.
[2] C. Amovilli, C. Filippi, F. M. Floris, J. Chem. Phys. (2008) 129 244106.
[3] F. M. Floris, C. Filippi, C. Amovilli, J. Chem. Phys. (2012) 137 075102.
[4] F. M. Floris, C. Filippi, C. Amovilli, J. Chem. Phys. (2014) 140 034109.
[5] R. Guareschi, C. Filippi, J. Chem. Theor. Comput. (2013) 9 5513.
[6] R. Guareschi, F. M. Floris, C. Amovilli, C. Filippi, in preparation (2014)
Shannon entropy and correlation energy for electrons in atoms
No full textIn this work, we compute Shannon entropy, defined in terms of electron density, for three series of atomic ions including the region of nuclear charges close to the limit at which the ionization potential goes to zero.We use both Hartree-Fock (HF) and quantum Monte Carlo (QMC) densities and we observe a sharp positive deviation ofQMCentropy with respect to theHF corresponding value in approaching the limit.We discuss this behaviour taking into account Coulomb correlation, which plays an important role in the weak binding regime
Shannon Entropy in Atoms: A Test for the Assessment of Density Functionals in Kohn-Sham Theory
Full text linkElectron density is used to compute Shannon entropy. The deviation from the Hartree–Fock (HF) of this quantity has been observed to be related to correlation energy. Thus, Shannon entropy is here proposed as a valid quantity to assess the quality of an energy density functional developed within Kohn–Sham theory. To this purpose, results from eight different functionals, representative of Jacob’s ladder, are compared with accurate results obtained from diffusion quantum Monte Carlo (DMC) computations. For three series of atomic ions, our results show that the revTPSS and the PBE0 functionals are the best, whereas those based on local density approximation give the largest discrepancy from DMC Shannon entropy
Method to Compute the Solute–Solvent Dispersion Contribution to the Electronic Excitation Energy in Solution
Full text link[Image: see text] A method formulated within the polarizable continuum model of the solvent and a quantum Monte Carlo treatment of the electronic states of the solute molecule is presented for the calculation of the solute–solvent dispersion contribution to the electronic excitation energy in solution. Variational quantum Monte Carlo is exploited to measure the fluctuations of the electronic electric field of the solute molecule to compute the London’s dispersion forces with the solvent. The method previously applied to the ground state of the solute is here extended to excited states. To perform the Casimir–Polder integration, we introduce a positive parameter Ω whose value is properly chosen for this purpose. We derive a general expression that for Ω = 0 reduces to that already proposed for the ground state. For an excited state, Ω must be less than the first transition electronic energy of the solvent molecule but greater than the transition energy from the ground to excited electronic state of the solute molecule. Benchmark calculations were performed on the n → π* transition for formaldehyde, acrolein, and acetone in six solvents, including water, ethanol, cyclohexane, chloroform, carbon tetrachloride, and toluene, and the π → π* transition of acrolein in cyclohexane. Solvents are characterized by their ionization potential and the refractive index at frequency Ω. In all cases, we found that the dispersion solute–solvent interaction stabilizes the excited state of the solutes leading to red (negative) solvatochromic shifts
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