Relativistic effective valence shell Hamiltonian method: Excitation and ionization energies of heavy metal atoms

Abstract

The relativistic effective valence shell Hamiltonian H(v) method (through second order) is applied to the computation of the low lying excited and ion states of closed shell heavy metal atoms/ions. The resulting excitation and ionization energies are in favorable agreement with experimental data and with other theoretical calculations. The nuclear magnetic hyperfine constants A and lifetimes tau of excited states are evaluated and they are also in accord with experiment. Some of the calculated quantities have not previously been computed.

Improved virtual orbital multireference Møller–Plesset study of the ground and excited electronic states of protonated acetylene, C2H3+

“…Recently, we have proposed a computationally inexpensive version of MRMP/MCQDPT in which the first order reference functions are generated from the improved virtual orbital complete active space configuration interaction ͑IVO-CASCI͒ method [33][34][35][36][37] and then are used in subsequent MR perturbation calculations. The IVO-CASCI scheme is computationally simpler than CI-singles ͑CIS͒ and complete active space SCF ͑CASSCF͒ methods.…”

Application of improved virtual orbital based multireference methods to N2, LiF, and C4H6 systems

“…Recently, Chaudhuri et al 16,17 proposed a computationally inexpensive version of HNPT in which the first order reference functions are generated from the IVO-CASCI method 15,[18][19][20][21] and are then used in subsequent MRPT calculations. The IVO-CASCI scheme is computationally simpler than CI singles ͑CISs͒ and CASSCF methods.…”

Investigation of Multiple-Bond Dissociation Using Brillouin–Wigner Perturbation with Improved Virtual Orbitals

“…Like other MR ab initio methods, the MR generalization of the perturbation theory is not obvious and a multitude of versions exist. − The development of various MRPT methods have contributed greatly to our understanding of molecular electronic spectra and other properties. The usefulness of the MRPT method (based on the distinction between nondynamical (or static) and dynamical correlation effects) can be extended using the improved virtual orbital (IVOs) complete active space configuration interaction (CASCI) technique (IVO-CASCI) , to generate a reference function. Our recently suggested state-specific (SS) Brillouin–Wigner MRPT (BWMRPT) with IVOs [termed IVO-BWMRPT], treating all components of the model space on the same footing, offers a useful and efficient ab initio protocol for studying near-degenerate states at an affordable computational cost.…”

“…4,5 Note that for the same size of the active space the IVO-CASCI protocol (even without IVO) is computationally cheaper than the CASSCF (complete active space self-consistent field) and allows one to access larger active spaces easily compared to the CASSCF because the CASCI does not perform orbital optimization. 53,54 Moreover, IVO-CASCI is free from the features that create multiple solutions, convergence difficulties, and unphysical spatial symmetry breaking of the wave function, which are often associated with the traditional MR-based mean-field calculations. Although the CASSCF is a workhorse for the study of MR situations, there is considerable interest in finding a useful and simpler alternative to it.…”

“…Although most MRPTs are beset by numerical divergence due to the intruder states problem (spoil computations with unphysical contributions problematic to heal systematically), − the IVO-BWMRPT methodology circumvents this obstacle in a natural and size-extensive manner because the IVO modification of the state-specific BWMRPT scheme retains all the advantages of the parent SSMR protocol of Mukherjee et al , that make it a serious candidate for efficient description of the dissociation of molecular bonds correctly and to compute global energy surfaces. Here, it is worth remarking that among the single-root/SS MRPTs, which are not based on the IVO-CASCI-like reference functions, an explicit intruder free scheme has also been proposed by Angeli et al [second-order n -electron valence state perturbation theory, NEVPT2]. , Note that for the same size of the active space the IVO-CASCI protocol (even without IVO) is computationally cheaper than the CASSCF (complete active space self-consistent field) and allows one to access larger active spaces easily compared to the CASSCF because the CASCI does not perform orbital optimization. , Moreover, IVO-CASCI is free from the features that create multiple solutions, convergence difficulties, and unphysical spatial symmetry breaking of the wave function, which are often associated with the traditional MR-based mean-field calculations. Although the CASSCF is a workhorse for the study of MR situations, there is considerable interest in finding a useful and simpler alternative to it. − It is worth mentioning that in the development and implementation of the MRPT method, a trade-off must be found between the price to pay to construct the MR mean-field like wave function and the cost required to compute second-order energy.…”