## Many-body perspectives on stochastic configuration interactions

**Seiichiro L. Ten-no**

Graduate School of Science, Technology, and Innovation, Kobe University, Kobe, Japon

Vendredi 23 Février 2018, 11h00

bibliothèque LCT, tour 12 - 13, 4^{ème} étage

Model space quantum Monte Carlo (MSQMC) [1,2] is an extension of full configuration interaction QMC (FCIQMC), that allows us to calculate quasi-degenerate and excited electronic states by sampling the effective Hamiltonian in the model space. We introduce a novel algorithm [3] based on the state-selective partitioning for the effective Hamiltonian using left eigenvectors to efficiently calculate several electronic states simultaneously. The sampling of walkers in MSQMC is analyzed in the single reference limit using a stochastic algorithm for higher-order perturbation energies by the analogy of the deterministic case utilizing a full configuration interaction program. We further develop size-consistency corrections of the initiator adaptation (i-MSQMC), *i.e.* the coupled electron pair approximation, a posteriori, and second-order perturbative corrections. It is clearly demonstrated that most of the initiator error is originating from the deficiency of proper scaling of correlation energy due to its truncated CI nature of the initiator approximation, and that the greater part of the error can be recovered by the size-consistency corrections.

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**References :**

[1] S. Ten-no *J. Chem. Phys.* **2013**, *138*, 164126.

[2] Y. Ohtsuka and S. Ten-no *J. Chem. Phys.* **2015**, *143*, 214107.

[3] S. L. Ten-no *J. Chem. Phys.* **2017**, *147*, 244107.