Open‐shell systems investigated with Monte Carlo configuration interaction

Abstract

We investigate the open‐shell systems of the methyl radical, the allyl radical and molecular oxygen using Monte Carlo configuration interaction. We look at whether modifying Monte Carlo configuration interaction to use the increased flexibility of unrestricted Hartree‐Fock orbitals offers any benefits in the description of these systems by the resulting compact wavefunctions. The expectation of the total spin squared when using Slater determinants is calculated to investigate if pure spin states can be evolved by Monte Carlo configuration interaction and how this might be accelerated. We consider the multireference character of these open‐shell systems. To this end we demonstrate how a previously introduced way [J. P. Coe and M. J. Paterson, J. Chem. Theory Comput. 11, 4189 (2015)] of viewing a multireference indicator of P.‐O. Löwdin [P.‐O. Löwdin, Phys. Rev. 97, 1474 (1955)] in terms of the spatial entanglement can be generalized to open‐shell systems.

Machine Learning Configuration Interaction for ab Initio Potential Energy Curves

“…the time cost is not too important for individual MR calculations so we make the CSFs exactly orthonormal for use with this indicator. To do this we use the approach of Ref 63,. which means that the new coefficients are c new = √ S c where S is the overlap matrix for the original non-orthonormal CSFs.…”

Investigation of challenging spin systems using Monte Carlo configuration interaction and the density matrix renormalization group

“…We are interested in the challenge that these spin systems represent when using the computationally accessible HF orbitals. Hence, we use MR , rather than an orbital invariant approach, to quantify the multireference character although we note that an orbital invariant approach has been generalized to open‐shell systems . Furthermore we use nonorthonormal CSFs to calculate MR , but note that a computationally intensive transformation of the CSFs to an orthonormal basis for calculating MR was used in Ref.…”

“…Hence, we use MR, rather than an orbital invariant approach, to quantify the multireference character although we note that an orbital invariant approach has been generalized to open-shell systems. [37] Furthermore we use nonorthonormal CSFs to calculate MR, but note that a computationally intensive transformation of the CSFs to an orthonormal basis for calculating MR was used in Ref. [37] and we investigate this approach on TiCO in this work.…”

“…We also use the procedure from Ref. [37] to calculate MR values when the CSFs are made to be orthonormal to illustrate comparison with MR values for Slater determinant wavefunctions. [38] Here, we find 0.32 for the singlet, 0.26 for the triplet and 0.25 for the septet.…”

“…[26] The method has now encompassed potential energy surfaces for the ground state, [27] electronic excitations, [28] excited potential curves including avoided crossings or conical intersections, [29] multipole moments, [30] potential curves of transition metal dimers, [31] polarizabilities and hyperpolarizabilities, [32] dissociation with perturbation corrections, [33] tunnel junctions comprised of gold atoms, [34] X-ray absorption and emission, [35] positronic systems, [36] and, most recently, open-shell molecules. [37] In the latter work, MCCI was enhanced to calculate the spin state and use unrestricted HF orbitals when working with Slater determinants. However, no significant improvement was observed when using unrestricted HF orbitals.…”