Columbus—a program system for advanced multireference theory calculations

Abstract

The COLUMBUS Program System allows high-level quantum chemical calculations based on the multiconfiguration self-consistent field, multireference configuration interaction with singles and doubles, and the multireference averaged quadratic coupled cluster methods. The latter method includes size-consistency corrections at the multireference level. Nonrelativistic (NR) and spin-orbit calculations are available within multireference configuration interaction (MRCI). A prominent feature of COLUMBUS is the availability of analytic energy gradients and nonadiabatic coupling vectors for NR MRCI. This feature allows efficient optimization of stationary points and surface crossings (minima on the crossing seam). Typical applications are systematic surveys of energy surfaces in ground and excited states including bond breaking. Wave functions of practically any sophistication can be constructed limited primarily by the size of the CI expansion rather than by its complexity. A massively parallel CI step allows state-of-the art calculations with up to several billion configurations. Electrostatic embedding of point charges into the molecular Hamiltonian gives access to quantum mechanical/molecular mechanics calculations for all wave functions available in COLUMBUS. The analytic gradient modules allow on-the-fly nonadiabatic photodynamical simulations of interesting chemical and biological problems. Thus, COLUMBUS provides a wide range of highly sophisticated tools with which a large variety of interesting quantum chemical problems can be studied. C 2011 John

Photoionization using the xchem approach: Total and partial cross sections of Ne and resonance parameters above the 2s22p5 threshold

“…(17), then we could get the branching ratios from (19). The problem lies now on how to perform the fitting.…”

“…In contrast, for bound molecular states, electron correlation can be accurately handled by using a variety of quantum chemistry packages (QCP) based on ab initio methods [16][17][18][19][20][21][22]. With proper adjustments, these methods can also provide an accurate description of molecular resonances (hole, shake-up, and multiply excited states), for which electron correlation is even more important [23][24][25][26].…”

Ultrafast photoinduced dynamics of halogenated cyclopentadienes: observation of geminate charge-transfer complexes in solution

“…[52][53][54]59 For CASSCF calculations, the COLUMBUS V5.9 (ref. [60][61][62][63] program package was employed.…”

“…S12 in the ESI †). Since CC2 is known to poorly treat excited states with doubly excited character 64 and the second excited state of C 5 H 6 is known to have a multiconfigurational character with considerable contribution of a LUMO ' HOMO double excitation, the CC2 results are re-evaluated by recalculating the same interpolated path for C 5 Cl 6 employing SA-4-CASSCF(6,5)/6-31G* [60][61][62][63] (see Fig. S11 and a description of the employed CAS in the ESI †).…”

Communication: Unambiguous comparison of many-electron wavefunctions through their overlaps

“…To evaluate this hypothesis, the T 1 and T 2 states are computed at five different levels of theory using various complete active space self-consistent field (CASSCF) and multireference configuration interaction (MR-CI) expansions. [35][36][37][38] First, an active space of 6 electrons in 5 orbitals and the ANO-RCC-VDZP basis set (abbreviated D) 39 are chosen and the maximum excitation level is varied using the CASSCF(6,5), MR-CIS (6,5), and MR-CISD(6,5) levels of theory. Then the active space is increased to the MR-CISD (8,7) level, and the basis set is enlarged to the triple-ζ level (MR-CISD(6,5)/T).…”