An efficient representation of molecular correlated wave functions is proposed, which features regularization of the Coulomb electron–electron singularities via the F12-style explicit correlation and a pair-natural orbital factorization of the correlation components of the wave function expressed in the real space. The pair-natural orbitals are expressed in an adaptive multiresolution basis and computed directly by iterative variational optimization. The approach is demonstrated by computing the second-order Moller–Plesset energies of small- and medium-sized molecules. The resulting MRA-PNO-MP2-F12 method allows for the first time to compute correlated wave functions in a real-space representation for systems with dozens of atoms (as demonstrated here by computations on alkanes as large as C10H22), with precision exceeding what is achievable with the conventional explicitly correlated MP2 approaches based on the atomic orbital representations.
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We follow the notation used in previous works denoting the particles onto which an operator acts as subscripts, i.e., . For one-particle equations, the subscripts are dropped for brevity.
In this work, we only focus on the electron–electron cusp. The electron–nuclear cusp also plays a role in the real-space many-body methods since its seam has the same dimension as that of the electron–electron cusp. Although it is possible to regularize the electron–nuclei cusps via the NEMO approach described in Refs. 15 and 16, this is less crucial for the PNO-based approach we pursue here. It is because the cusps in PNOs are pointwise, and just like the cusps in the Hartree–Fock orbitals, these can be effectively described by the adaptive multiresolution refinement of the basis.
The number of iterations in the Hartree–Fock calculation did not change significantly.