Half-Projection Of Doubly-Occupied Configuration Interaction Wave Function

J. GARCIA; O.B. OÑA; D.R. ALCOBA; A. TORRE; L. LAIN; G.E. MASSACCESI

La Plata

Workshop; X Workshop On Novel Methods For Electronic Structure Calculations; 2023

UNIVERSIDAD NACIONAL DE LA PLATA

The doubly-occupied configuration interaction (DOCI) method expresses the N-electron system wave function in terms of doubly occupied Slater determinant basis sets [1]. It has been proven that the DOCI method captures most of the static correlation and, consequently, it has been proposed as an alternative to excitation-based truncations of the full configuration interaction (FCI) treatment. The energies of the DOCI Hamiltonian depend on the single-particle basis set employed, and therefore a minimization with respect to orbital rotations is required. If this optimization is carried out by performing the same transformation on the α- and β- orbitals (restricted DOCI, or RDOCI), one obtains a good description of molecules near equilibrium distances, but the dissociation products are not predicted correctly. On the other hand, if the optimization is performed by allowing the α- and β- orbitals to be different (unrestricted DOCI, or UDOCI), the description of molecular dissociation, in general, is correct but at the cost of global spin contamination [2, 3]. The half-projection method consists in projecting the odd or even spin quantum number components out of a wavefunction by combining it linearly with its spin conjugate [4]. For singlet states, this projection usually leads to a better wavefunction since the spin contamination from triplet contributions, which is usually the most relevant, is removed. In the present work we aim to correct the spin contamination of UDOCI wave functions by means of this method, by performing the half-projection both before and after orbital optimization. We employ as a test case scenario the dissociation curves of several H4 clusters, and show that, for these systems, spin contamination is almost removed, while the energies are improved.[1] L. Bytautas, T. M. Henderson, C. A. Jiménez-Hoyos, J. K. Ellis, and G. E. Scuseria, J. Chem. Phys. 135, 44119 (2011). [2] D. R. Alcoba, A. Torre, L. Lain, G. E. Massaccesi, O. B. O?na, and E. R´ıos, J. Chem. Phys. 150, 164106 (2019). [3] J. Garcia, O. B. O?na, A. Torre, L. Lain, D. R. Alcoba, and G. E. Massaccesi, Int. J. Quantum Chem. 123, e27119 (2023). [4] Y. G. Smeyers and L. Doreste-Suarez, Int. J. Quantum Chem. 7, 687 (1973).