Density of States Guided Møller-Plesset Perturbation Theory.

An integral formalism using a density-of-state framework has been developed for Møller-Plesset perturbation theory. This method is designed to compute the correlation energy correction for large systems with high density of states, such as polymers and nanostructures. The framework has the potential to lower the computational cost of perturbation theory, and such perspectives are discussed in this paper.

Projected Coupled Cluster Amplitudes from a Different Basis Set As Initial Guess.

Many model chemistry schemes require a sequence of high level calculations, often at the coupled cluster (CC) level, with increasingly larger basis sets. The CC equations are solved iteratively, and the rate of convergence strongly depends on the quality of the initial guess. Here, we propose a strategy to define a better guess from a previous CC calculation with a different basis set by employing the concept of corresponding orbitals.

Oscillator Strength: How Does TDDFT Compare to EOM-CCSD?

In this work, we compare a large variety of density functionals against the equation of motion coupled cluster singles and doubles (EOM-CCSD) method for the calculation of oscillator strengths. Valence and Rydberg states are considered for a test set composed of 11 small organic molecules. In our previous work, the same systems and methods were tested against experimental results for the excitation energies.

Oscillator Strengths in ONIOM Excited State Calculations.

We compute oscillator strengths with the ONIOM (Our own N-layer Integrated molecular Orbital molecular Mechanics) hybrid method between ground and valence excited states and compare the results with the high level of theory equation of motion coupled cluster singles and doubles (EOM-CCSD). This work follows our previous studies in which we validated the ability of ONIOM to compute accurate transition energies compared to EOM-CCSD. We test various levels of theory and molecular systems, as well as the effect of the link atom bond length.

Link atom bond length effect in ONIOM excited state calculations.

We investigate how the choice of the link atom bond length affects an electronic transition energy calculation with the so-called our own N-layer integrated molecular orbital molecular mechanics (ONIOM) hybrid method. This follows our previous paper [M. Caricato et al.

A Comparison of Three Variants of the Generalized Davidson Algorithm for the Partial Diagonalization of Large Non-Hermitian Matrices.

The solution of the equation of motion coupled cluster singles and doubles problem, that is finding the lowest lying electronic transition energies and properties, is fundamentally a large non-Hermitian matrix diagonalization problem. We implemented and compared three variants of the widely diffuse generalized Davidson algorithm, which iteratively finds the lowest eigenvalues and eigenvectors of such a matrix. Our numerical tests, based on different molecular systems, basis sets, state symmetries, and reference functions, demonstrate that the separate evaluation of the left- and right-hand eigenvectors is the most efficient strategy to solve this problem considering storage, numerical stability, and convergence rate.

Electronic excitation energies in solution at equation of motion CCSD level within a state specific polarizable continuum model approach.

We present a study of excitation energies in solution at the equation of motion coupled cluster singles and doubles (EOM-CCSD) level of theory. The solvent effect is introduced with a state specific polarizable continuum model (PCM), where the solute-solvent interaction is specific for the state of interest. Three definitions of the excited state one-particle density matrix (1PDM) are tested in order to gain information for the development of an integrated EOM-CCSD/PCM method.

Electronic Transition Energies: A Study of the Performance of a Large Range of Single Reference Density Functional and Wave Function Methods on Valence and Rydberg States Compared to Experiment.

This work reports a comparison among wave function and DFT single reference methods for vertical electronic transition energy calculations toward singlet states, valence and Rydberg in nature. A series of 11 small organic molecules are used as test cases, where accurate experimental data in gas phase are available. We compared CIS, RPA, CIS(D), EOM-CCSD, and 28 multipurpose density functionals of the type LSDA, GGA, M-GGA, H-GGA, HM-GGA and with separated short and long-range exchange.

On the difference between the transition properties calculated with linear response- and equation of motion-CCSD approaches.

In this work, we quantitatively investigate the difference between the linear response (LR) and the equation of motion (EOM) coupled cluster (CC) approaches in the calculation of transition properties, namely, dipole and oscillator strengths, for the most widely used truncated CC wave function, which includes single and double excitation operators. We compare systems of increasing size, where the size-extensivity may be important. Our results suggest that, for small molecules, the difference is small even with large basis sets.

Using the ONIOM hybrid method to apply equation of motion CCSD to larger systems: benchmarking and comparison with time-dependent density functional theory, configuration interaction singles, and time-dependent Hartree-Fock.

Equation of motion coupled-cluster singles and doubles (EOM-CCSD) is one of the most accurate computational methods for the description of one-electron vertical transitions. However, its O(N(6)) scaling, where N is the number of basis functions, often makes the study of molecules larger than 10-15 heavy atoms prohibitive. In this work we investigate how accurately less expensive methods can approximate the EOM-CCSD results.

Permanganate oxidation of alkenes. Substituent and solvent effects. Difficulties with MP2 calculations.

The permanganate oxidation of alkenes has been studied both experimentally and computationally. Transition state structures were located for the reaction of permanganate ion with a variety of monosubstituted alkenes at the B3LYP/6-311++G** level. Although the calculated activation energy for the reaction with ethene was reasonable, the calculated effect of substituents, based on the energies of the reactants, was much larger than that experimentally found.