State-averaged Monte Carlo configuration interaction applied to electronically excited states

Abstract

We introduce state-averaging into the method of Monte Carlo configuration interaction (SA-MCCI) to allow the stable and efficient calculation of excited states. We show that excited potential curves for H 3 , including a crossing with the ground state, can be accurately reproduced using a small fraction of the FCI space. A recently introduced error measure for potential curves [J. P. Coe and M. J. Paterson, J. Chem. Phys., 137, 204108 (2012)] is shown to also be a fair approach when considering potential curves for multiple states. We demonstrate that potential curves for LiF using SA-MCCI agree well with the FCI results and the avoided crossing occurs correctly. The seam of conical intersections for CH 2 found by Yarkony [J. Chem. Phys., 104, 2932 (1996)] is used as a test for SA-MCCI and we compare potential curves from SA-MCCI with FCI results for this system for the first three triplet states. We then demonstrate the improvement from using SA-MCCI on the dipole of the 2 1 A 1 state of carbon monoxide. We then look at vertical excitations for small organic molecules up to the size of butadiene where the SA-MCCI energies and oscillator strengths are compared with CASPT2 values [M. Schreiber, M. R. Silva-Junior, S. P. A. Sauer, and W. Thiel, J. Chem. Phys., 128, 134110 (2008)]. We finally see if the SA-MCCI results for these excitation energies can be improved by using MCCIPT2 with approximate natural orbitals when the PT2 space is not onerously large.

Approaching exact hyperpolarizabilities via sum-over-states Monte Carlo configuration interaction

“…We use state-averaged MCCI (SA-MCCI) 7 for a small number of excited states to produce a tractable set of configurations that aims to capture enough of the ground and excited state aspects of the FCI wavefunction. We then investigate the use of these configurations in computationally viable sumover-states calculations for dipole polarizabilities, hyperpolarizabilities, and second hyperpolarizabilities.…”

Multireference X-ray emission and absorption spectroscopy calculations from Monte Carlo configuration interaction

“…MCCI stochastically builds up a wavefunction with the aim of capturing many of the important aspects of the FCI wavefunction by accounting for both static and dynamic correlation to some degree, but using only a very small fraction of configurations. The method has been successfully applied to single point energies [18], dissociation energies [19,20], electronic excitations [21,22], ground-state [23,24] and excited potential curves [22], multipole moments [25] and higher-order dipole properties up to the second hyperpolarizability [26].…”

Open‐shell systems investigated with Monte Carlo configuration interaction

“…The calculation does not depend on a restriction of the configuration space using chemical intuition. MCCI has previously been applied to single point energies, dissociation energies, and electronic excitations . Ground‐state potential curves were investigated in Ref.…”