01630nas a2200181   4500008004100000022001400041245010700055210006900162260007100231300001400302490000700316520092200323100002701245700002001272700001801292700001601310856012201326       2014                        eng d  a1549-961800aIntermediate hamiltonian fock space multireference coupled cluster approach to core excitation spectra0 aIntermediate hamiltonian fock space multireference coupled clust  a1155 16TH ST, NW, WASHINGTON, DC 20036 USAbAMER CHEMICAL SOCcSEP  a3656-36680 v103 a<p>The Fock space multireference coupled cluster (FSMRCC) method provides an efficient approach for the direct calculation of excitation energies. In intermediate Hamiltonian (IH-FSMRCC) formulation, the method is free from intruder state problems and associated convergence difficulties, even with a large model space. In this paper, we demonstrate that the IH-FSMRCC method with suitably chosen model space can be used for the accurate description of core excitation spectra of molecules, and our results are in excellent agreement with the experimental values. We have investigated the effect of choice of model space on the computed results. Unlike the equation-of-motion (EOM)-based method, the IH-FSMRCC does not require any special technique for convergence and in singles and doubles approximation gives a performance comparable to that of the standard EOMEE-CCSD method, even better in some of the cases.</p>1 aDutta, Achintya, Kumar1 aGupta, Jitendra1 aVaval, Nayana1 aPal, Sourav  uhttps://library.ncl.res.in/content/intermediate-hamiltonian-fock-space-multireference-coupled-cluster-approach-core-0