We used the atomic natural orbital (ANO-L) basis set46 of polarised triple-zeta (double-zeta for hydrogen) quality (TZVP (14s9p4d3f)/[4s2p2d1f] for carbon and oxygen, DZVP (8s4p3d)/[2s1p] for hydrogen). To reduce computational time, we used the Cholesky decomposition47 with the default parameters for computation of two-electron integrals.