Efficient calculations of a large number of highly excited states for multiconfigurational wavefunctions

Abstract

Electronically excited states play important roles in many chemical reactions and spectroscopic techniques. In quantum chemistry, a common technique to solve excited states is the multiroot Davidson algorithm, but it is not designed for processes like X‐ray spectroscopy that involves hundreds of highly excited states. We show how the use of a restricted active space wavefunction together with a projection operator to remove low‐lying electronic states offers an efficient way to reach single and double‐core‐hole states. Additionally, several improvements to the stability and efficiency of the configuration interaction (CI) algorithm for a large number of states are suggested. When applied to a series of transition metal complexes the new CI algorithm does not only resolve divergence issues but also leads to typical reduction in computational time by 70%, with the largest savings for small molecules and large active spaces. Together, the projection operator and the improved CI algorithm now make it possible to simulate a wide range of single‐ and two‐photon spectroscopies. © 2019 Wiley Periodicals, Inc.

Vibrational wavepacket dynamics in Fe carbene photosensitizer determined with femtosecond X-ray emission and scattering

“…All structures belong to the D2 point group but as the iron is six-coordinated, Oh point group labels will be used to describe the metal orbitals. The spectra were calculated using the ab initio Restricted Active Space (RAS) approach in the OpenMolcas package (version v18.0.o180105-1800) [69], using a new efficient configuration interaction algorithm [70]. The electronic structure of the valence and coreionized electronic states was evaluated at RASSCF/ANO-RCC-VDZP level of theory [71][72][73].…”

OpenMolcas: From Source Code to Insight

“…13. 269 Skipping the calculation of correction vectors for the converged low-lying states not only reduces significantly the computational time but also the number of iterations. The reason is that as the lowest states converge, more and more of the vectors used as basis for the Davidson diagonalization correspond to the remaining higher states, which can thus converge faster.…”

Simulations of valence excited states in coordination complexes reached through hard X-ray scattering

“…The calculations were made possible by a new stable and efficient algorithm for CI calculations with a large number of states, which is now the default algorithm in OpenMolcas. 70 For detailed information about the number of states in different calculations, see Tables SI-2…”

“…To reach the high-energy core-excited states, a projection technique is used to remove all configurations with doubly-occupied core orbitals in the intermediate state (HEXS keyword). 45,70 To avoid orbital rotation, i.e., that the hole appears in a higher-lying orbital, the 1s orbital was not re-optimized in the core-excited states. As the spin-orbit coupling remains relatively weak during the RIXS process, only intermediate and final states of the same spin multiplicity as the initial state have been included.…”