A new geminal ansatz called antisymmetrized product of 2D-block geminals has been introduced recently to solve the electronic Schr¨odinger equation for molecules [1]. It builds on an antisymmetrized product of strongly orthogonal geminals (APSG) while lifting the “strong orthogonality” and “seniority zero” restrictions. The combinatorics to select the single excitations which can be added to each geminal of the product, grows exponentially with the number of geminals. We will present simple guidelines which drastically reduce the number of useful excitations able to lower significantly the electronic energy in variational calculations. Although different regimes to partition in an optimal way the orbital space into 1D- and 2D-blocks, are observed along the potential energy curves (PEC), the same strategy to choose the block types associated to the geminals in each 2D-block, is found to apply successfully in all cases. This makes the method quite practical to use. Moreover, in most cases, the geminals can be easily related to classical Lewis structures of chemistry. The quality of the PECs is assessed by calculating equilibrium geometries and fundamental frequencies, the improvement with respect to APSG is discussed.

References

[1] P. Cassam-Chenaï, Thomas Perez, Davide Accomasso, J. Chem. Phys 158, 074106 (2023).