Dyall Hamiltonian
In quantum chemistry, the Dyall Hamiltonian is a modified Hamiltonian with two-electron nature. It can be written as follows:
where labels,, denote core, active and virtual orbitals respectively, and are the orbital energies of the involved orbitals, and operators are the spin-traced operators. These operators commute with and, therefore the application of these operators on a spin-pure function produces again a spin-pure function.
The Dyall Hamiltonian behaves like the true Hamiltonian inside the CAS space, having the same eigenvalues and eigenvectors of the true Hamiltonian projected onto the CAS space.
