A program for the direct calculation of excitation energies of atoms and molecules in strong magnetic fields is presented. The implementation includes the equation-of-motion coupled-cluster singles-doubles (EOM-CCSD) method for electronically excited states as well as its spin-flip variant. Differences to regular EOM-CCSD implementations are due to the appearance of the canonical angular-momentum operator in the Hamiltonian causing the wave function to become complex. The gauge-origin problem is treated by the use of gauge-including atomic orbitals. Therefore, a modified Davidson method for diagonalizing complex non-Hermitian matrices is used. Excitation energies for selected atoms and molecules that are of importance in the astrochemical context are presented and their dependence on the magnetic field is discussed.
REFERENCES
Both eigenvectors are needed to compute transition probabilities, see Ref. 23.
Note that complex matrix-matrix multiplications are performed by using the BLAS-like extension routine zgemm3m. This routine only performs three real matrix multiplications and five real matrix additions instead of the conventional four matrix multiplications and two additions (see https://software.intel.com/en-us/node/468652). Therefore, the cost is roughly three instead of four times higher.
