Accurately solving the electronic Schrodinger equation of small atoms and molecules using explicitly correlated (r12-)MR-CI. VIII. Valence excited states of methylene (CH2).

We compute the adiabatic transition energies of methylene (CH(2)) from the ground state to the lowest electronically excited valence states using the r(12)-MR-ACPF-2 method with a large basis set and an extended reference space. We recall that this method aims at reaching the basis-set and full configuration interaction (CI) limits simultaneously. Our best excitation energies, T(e) (T(0)), are 9.

Accurately solving the electronic Schrodinger equation of atoms and molecules using explicitly correlated (r12-) multireference configuration interaction. VII. The hydrogen fluoride molecule.

We compute the potential-energy curve of the hydrogen fluoride molecule (HF) using a novel variant of the explicitly correlated multireference averaged coupled-pair functional method with a carefully selected basis set and reference space. After correcting for scalar relativistic effects and spin-orbit coupling, the potential is used to compute the dissociation energy, the equilibrium bond distance, the harmonic frequency, the anharmonicity, and the vibrational levels up to the dissociation limit. The errors in the equilibrium geometry constants compare favorably with the most elaborate (single reference) calculations of the literature.

Registering the Amica electronic structure code in the extensible computational chemistry environment.

We describe the integration and use of the Amica software package ("Atoms & Molecules In Chemical Accuracy") within the Extensible Computational Chemistry Environment (Ecce). Amica is capable of accurately solving the electronic Schrodinger equation of small atoms and molecules using terms that are linear in the interelectronic distances, r(12), on multireference level of theory, but it requires expert knowledge to configure and execute its algorithms. Ecce is a comprehensive suite of tools that support the computational chemistry research processes of computation setup, execution, and analysis through a convenient graphical user interface.

Size extensive modification of local multireference configuration interaction.

We recently developed a reduced scaling multireference configuration interaction (MRCI) method based on local correlation in the internal (occupied) and external (virtual) orbital spaces. This technique can be used, e.g.