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.

12 pages, 9 figures

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• Stochastic determination of effective Hamiltonian for the full configuration interaction solution of quasi-degenerate electronic states
• Excited States, Dynamic Correlation Functions and Spectral Properties from Full Configuration Interaction Quantum Monte Carlo
• Development of Monte Carlo configuration interaction: Natural orbitals and second-order perturbation theory
• Calculations of Potential Energy Surfaces Using Monte Carlo Configuration Interaction
• Monte Carlo configuration interaction applied to multipole moments, ionisation energies and electron affinities

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