Let me ask about orbital reordering in DMRG calculations.
Usually, the active space in a DMRG calculation is defined by using RESTRICTED_DOCC and ACTIVE.
These are arrays giving the number of orbitals per irrep. These options are useful in the calculations of
highly-symmetric molecules, such as benzene, naphthalene, and so on. In these molecules,
the irreps of sigma-orbitals are different from those of pi-orbitals. Therefore, pi-orbitals are successfully
defined as active orbitals by using RESTRICTED_DOCC and ACTIVE.
However, in the calculations of molecules with low symmetry, these options are not always effective and
the orbitals should be reordered manually. For example, the orbitals are in the order of pi, sigma, sigma, pi, pi, pi; and
these orbitals have the same symmetry. To construct the active space using only pi orbitals, the orbitals should be
reordered to sigma, sigma, pi, pi, pi, pi; and the first two orbitals are restricted to be doubly occupied.
How can I carry out such reordering? The mcscf_rotate option is not effective in DMRG calculations.
Can I employ CASSCF- or RASSCF-wave function with reordered orbitals as the initial guess of the DMRG calculation?
I appreciate your help in advance.