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

Abstract

We propose using sum-over-states calculations with the compact wavefunctions of Monte Carlo configuration interaction to approach accurate values for higher-order dipole properties up to second hyperpolarizabilities in a controlled way. We apply the approach to small systems that can generally be compared with full configuration interaction (FCI) results. We consider hydrogen fluoride with a 6-31g basis and then look at results, including frequency dependent properties, in an aug-cc-pVDZ basis. We extend one calculation beyond FCI by using an aug-cc-pVTZ basis. The properties of an H4 molecule with multireference character are calculated in an aug-cc-pVDZ basis. We then investigate this method on a strongly multireference system with a larger FCI space by modelling the properties of carbon monoxide with a stretched geometry. The behavior of the approach with increasing basis size is considered by calculating results for the neon atom using aug-cc-pVDZ to aug-cc-pVQZ. We finally test if the unusual change in polarizability between the first two states of molecular oxygen can be reproduced by this method in a 6-31g basis.

Investigating Multireference Character and Correlation in Quantum Chemistry

“…An upper bound for I is M ln 4 when every type of occupation is equally likely for each orbital although this may not be a realisable maximum due to the constraint of particle number. We therefore also calculateĨ = I M ln 4 (26) and similarlyĨ nat . 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 3 Results…”

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

“…In Ref. [26] the value for HF in a aug-cc-pVDZ basis was found to be 0.30 for an MCCI wavefunction suggesting that this system is amenable to being modelled using methods based on a single reference. …”

“…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].…”

Frequency dispersion of the first hyperpolarizabilities of reference molecules for nonlinear optics

“…[1][2][3] Small systems constitute indeed ideal cases to assess the reliability of new quantum chemistry methods, tackling in particular electron correlation effects and vibrational contributions. [4][5][6][7][8][9][10][11][12][13][14][15] Moreover, these systems are used as references for experimental determinations, which rely on relative rather than on absolute measurements. 16 Several groups have worked out procedures to define reference β values for molecules in gas and liquid phases, [17][18][19][20][21] and successive improvements in these predictions have taken advantages of the synergy with the accuracy improvements in ab initio calculations.…”