Two-step method for precise calculation of core properties in molecules A.V.Titov, N.S.Mosyagin, A.N.Petrov, and T.A.Isaev Petersburg Nuclear Physics Institute RAS Gatchina, St.-Petersburg 188300, RUSSIA Precise calculations of ``core'' properties which are described by the operators heavily concentrated in atomic cores, like to hyperfine structure (HFS) and P,T-parity nonconservation (PNC) effects, usually require accounting for relativistic effects. Unfortunately, even completely relativistic calculations of diatomics containing elements from fourth period are very consuming already on the stages of calculation and transformation of two-electron integrals with a basis of four-component spinors. The complexity of such calculations becomes very high for compounds of d- and, especially, f-elements. In turn, the relativistic effective core potential (RECP) calculations of ``valence'' (spectroscopic, chemical etc.) properties of molecules are very popular now because the RECP method allows one to treat satisfactory the correlation and relativistic effects in the valence region of a molecule with minimal efforts. However, the accuracy of the conventional RECPs is limited [1]. Besides, the valence molecular spinors are usually smoothed in atomic cores and, as a result, direct calculation of electronic densities near heavy nuclei is impossible. The former circumstance stimulated further development of the RECP approaches and, in particular, the generalized RECP (GRECP) concept was proposed [1] employing the idea of different treatment of inner core, outer core and valence shells. The latter had led to the methods of nonvariational [2] and variational [3] one-center restoration of correct shapes of four-component spinors in atomic cores after a two-component (G)RECP calculation of a molecule. In the report, the restoration and correlation methods are discussed and their efficiency is illustrated in calculations of HFS and PNC effects in heavy-atom molecules [2]. The present work was supported by U.S. CRDF grant No. RP2-2339-GA-02 and RFBR grant No. 03-03-32335. N.M. is supported by the grants of the Leningrad district governor and Russian science support foundation. A.P. is grateful to the Ministry of Education RF (Grant PD02-1.3-236) and to St.Petersburg Committee on Science and HE (Grant PD03-1.3-60). T.I. thanks INTAS for Grant YSF 2001/2-164. [1] A.V.Titov, N.S.Mosyagin, Int.J.Quant.Ch. 71, 359 (1999); A.V.Titov, DSc Thesis (PNPI, 2002); see also http://qchem.pnpi.spb.ru. [2] A.V.Titov, PhD Thesis (Leningrad University, 1985); A.V.Titov, N.S.Mosyagin, V.F.Ezhov, Phys.Rev.Lett. 77, 5346 (1996); M.G.Kozlov, A.V.Titov, N.S.Mosyagin, Phys.Rev.A 56, R3326 (1997); N.S.Mosyagin, M.G.Kozlov, A.V.Titov, J.Phys.B 31, L763 (1998); A.N.Petrov et al., Phys.Rev.Lett. 88, 073001 (2002); T.A.Isaev et al., Phys.Rev.A (Rapid Comm.), in press. [3] A.V.Titov, Int.J.Quant.Chem. 57, 453 (1996).