Parallel implementation of the equation-of-motion coupled-cluster singlesand doubles method and application for radical adducts of cytosine

摘要

The equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) method has beenimplemented into the massively parallel ACES III program using two alternative strategies: (1) storingthe entire EOM Hamiltonian matrix prior to diagonalization and (2) recomputing the four-virtualpart of the matrix from integrals in a direct mode. The second is found to be far more efficient.EOM-CC shows virtually ideal scaling from 32 to 256 processors. With basis sets as large as 552functions, the program was applied to determine vertical excitation energies for five cytosine radicaladducts of –OH and –H at three sites C5, C6, and N3. These radicals are considered to play animportant role in radiation induced DNA damage. The excitation energy spectrum shows twodistinct patterns for the lowest transitions distinguishing the C6–OH, C6–H, and N3–H adductsfrom the C5-OH and C5–H. The results indicate that the two lowest transitions of the C6–OHisomer should contribute to the experimentally observed absorption maximum at 2.88 eV, while thethird and fourth transitions of C6–OH and the two lowest transitions of C5–OH contribute to the3.65 eV absorption maximum. We also report the CCSD with noniterative triples correction[CCSD(T)] relative energies of the C5-OH and C6–OH adducts using 1000 processors.