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

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

Study of correlation holes. II. CI calculations on model polyatomic systems

The shape of correlation holes in many-electron systems is at present scarcely known, even where correlated wave functions are available. We investigate here the kind of electron correlation brought about by configuration interaction (CI), within a given basis set, in the wavefunction of a polyatomic system. The model ring system H6 (in two different bonding circumstances) and H14 have been chosen for a detailed study, because of their paradigmatic importance. We set out the equal-spin and different-spin correlation holes as obtained from complete CI calculations in H6 and partial ct in H14, both within a minimal basis set. We basically find the spinless correlation as being short range, while the spin-dependent correlation holes show long-range oscillations of antiferromagnetic character. We also present a natural spin-geminal analysis of the two-body reduced density matrices in these systems; we find a peculiarity possibly related to the long-range correlation discussed above. Finally, we compare the electron correlation as given from our CI wavefunction to other pictures of electron correlation, as obtained essentially from alternant molecular orbital wave functions and from the electron–gas literature

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

Rotation and inversion states in thermal E/Z isomerization of aromatic azo compounds

Ab initio calculations in a variation—perturbation scheme have been carried out on the three molecules of diazene, phenyldiazene and azobenzene with a view to clarifying the Z/E isomerization mechanism. It is found that, contrary to the usual belief, the rotational isomerization mechanism shows an energy barrier quite comparable to the inversional one and cannot generally be discarded in the Z/E thermal conversion