Abstract

In this talk, I will present some of our recent advances on selected configuration interaction (sCI) and quantum Monte Carlo (QMC) methods. In particular, I will detail our protocol to deterministically construct nodal surfaces within QMC using the sCI algorithm CIPSI (Configuration Interaction using a Perturbative Selection made Iteratively) [1]. In contrast to standard QMC implementations, these nodes can be systematically and deterministically improved by increasing the size of the sCI expansion. The present methodology will be illustrated on the transition metal sulfide molecule FeS [2], which is particularly challenging due to the proximity of two low-energy electronic states of different spatial symmetry (see Fig. 1). Results on the excited states of small organic molecules will also be presented, and we are going to show that, by using small multideterminant expansions without Jastrow factor, one can obtain highly accurate vertical transition energies. I will also present a new way of introducing the explicit correlation between electrons within the configu- ration interaction (CI) method via a dressing of the CI matrix [3]. Compared to other explicitly-correlated methods, this dressing strategy has the advantage of introducing the explicit correlation at a relatively low computational cost [4]. The present idea is completely general and can be applied to any type of trun- cated, full, or even selected CI method. Figure 1: Deterministic construction of nodal surfaces within QMC for FeS [1] B. Huron, J. P. Malrieu, and P. Rancurel, J. Chem. Phys. 58, 5745 (1973). [2] A.Scemama,Y.Garniron,M.CaffarelandP.F.Loos,J.Chem.TheoryComput.14,1395(2018). [3] J.-L. Heuilly and J.-P. Malrieu, Chem. Phys. Lett. 199, 545 (1992). [4] G. M. J. Barca and P. F. Loos, J. Chem. Phys. 147, 024103 (2017).