A convenient decontraction procedure of internally contracted state-specific multireference algorithms

Internally contracted state-specific multireference MR algorithms, either perturbative such as CASPT2 or NEVPT2, or nonperturbative such as contracted MR configuration interaction or MR coupled cluster, are computationally efficient but they may suffer from the internal contraction of the wave function in the reference space. The use of a low dimensional multistate model space only offers limited flexibility and is not always practicable. The present paper suggests a convenient state-specific procedure to decontract the reference part of the wave function from a series of state-specific calculations using slightly perturbed zero-order wave functions. The method provides an orthogonal valence bond reading of the ground state and an effective valence Hamiltonian, the excited roots of which are shown to be relevant. The orthogonal valence bond functions can be considered quasidiabatic states and the effective valence Hamiltonian gives therefore the quasidiabatic energies and the electronic coupling among the quasidiabatic states. The efficiency of the method is illustrated in two case problems where the dynamical correlation plays a crucial role, namely, the LiF neutral/ionic avoided crossing and the F2 ground state wave functio

Photoionization of furan from the ground and excited electronic states

Here we present a comparative computational study of the photoionization of furan from the ground and the two lowest-lying excited electronic states. The study aims to assess the quality of the computational methods currently employed for treating bound and continuum states in photoionization. For the ionization from the ground electronic state, we show that the Dyson orbital approach combined with an accurate solution of the continuum one particle wave functions in a multicenter B-spline basis, at the density functional theory (DFT) level, provides cross sections and asymmetry parameters in excellent agreement with experimental data. On the contrary, when the Dyson orbitals approach is combined with the Coulomb and orthogonalized Coulomb treatments of the continuum, the results are qualitatively different. In excited electronic states, three electronic structure methods, TDDFT, ADC(2), and CASSCF, have been used for the computation of the Dyson orbitals, while the continuum was treated at the B-spline/DFT level. We show that photoionization observables are sensitive probes of the nature of the excited states as well as of the quality of excited state wave functions. This paves the way for applications in more complex situations such as time resolved photoionization spectroscopy

Conformation of 1,4-dihydropyridine — planar or boat-like?

AbstractThe geometry of the 1,4-dihydropyridine molecule was completely optimized employing three different ab initio basis sets (6–31 G*, 4–31 G, STO—3G). The most reliable 6–31G* basis set provides a very flat boat conformation which may easily undergo defolding to a planar ring arrangement. This result is discussed with respect to enzymatic redox cofactors and the pharmacological activity of dihydropyridine calcium antagonists

The treatment of electronically excited states with multireference perturbation theory

The n-electron valence state perturbation theory (NEVPT) belongs to the family of multireference perturbation theories. Starting from a CAS-CI zero order calculation, all the contracted double excitations are generated and a zero order Hamiltonian is built making use of Dyall's model Hamiltonian where all the bielectronic interactions between the active electrons are taken into account. The first order correction to the wave function is built as a summation of multireference functions each of which has an energy corresponding to a well defined physical process (e.g. an ionization involving the active electrons). The NEVPT approach presents some desirable characteristics such as a) invariance under orbital rotation in each of the three orbital classes (core, active and virtual), b) strict separability (size consistence) with respect to molecular dissociation, c) absence of intruder states. The theory can be applied to any solution of a CAS-CI calculation and is therefore well suited to the treatment of electronically excited states. The theory can be formulated either in a state specific approach or in a quasi-degenerate formalism. The latter is well suited in cases where the mixing of the configurations in the zero order wave function is poorly described in the variational procedure, as can occur in avoided crossings between ionic and covalent states or in excited states of mixed valence-Rydberg character. The theory is illustrated through a few significant test calculations

Many-body multireference Moeller-Plesset and Epstein-Nesbet perturbation theory: fast evaluation of second-order energy contributions

Multireference perturbation theory is examined in connection with the two partitions in the Møller - Plesset and Epstein - Nesbet schemes. The implementation of an efficient diagrammatic technique is described and two examples of application (diazene and the Cr2 molecule), involving large variational spaces, are provide

Second order perturbation correction to CI energies by use of diagrammatic techniques: an improvement to the CIPSI algorithm

A usual procedure to get a large fraction of the correlation energy consists in the evaluation of the second order perturbation contribution to the electronic energy by utilizing as zeroth order state a moderate size CI wave function (CIPSI algorithm). A scheme of calculation based on a hole‐particle formulation of the Hamiltonian, leading to a diagrammatic pattern quite similar to the one used for the one‐determinant case, is proposed and discussed

Adiabatic and Diabatic Basis Sets in molecular Calculations

The concept of diabatic basis is examined. The most common methods of building diabatic functions are reported, focussing on those methods which do not presuppose the previous calculation of the dynamic coupling functions

FRODO: a MuPAD program to calculate matrix elements between contracted wavefunctions

A symbolic program performing the Formal Reduction of Density Operators (FRODO) has been developed in the MuPAD computer algebra system with the purpose of evaluating the matrix elements of the electronic Hamiltonian between internally contracted functions in a complete active space (CAS) scheme. The program is illustrated making use of two meaningful examples