The minimum basis set hydrogen rectangular system (HRS), consisting of four atoms arranged in a rectangle, is examined using variety partitionings the Hamiltonian H for high order single and double reference perturbation computations. potential energy surface mapped out over range geometries which length L one side rectangle varied. Several criteria are derived governing necessary conditions perturbative convergence two-state systems, these useful explaining behavior HRS partitioning methods...

We describe a computationally efficient ab initio many-body method that can be used as “packageable approximation” for computing excited state properties small to large molecular systems, including those of multiconfigurational character. The is based on first order multi-reference perturbation theory (MR-MBPT), where the unoccupied valence orbitals are obtained by using an extension Huzinaga’s improved virtual orbital (IVO) generation technique. Because employs complete active space (CAS)...

We present a new hybrid method to solve the relativistic Hartree-Fock-Roothan equations where one- and two-electron radial integrals are evaluated numerically by defining basis functions on grid. This procedure reduces computational costs in evaluation of integrals. The orbitals generated this employed compute ionization potentials, excitation energies oscillator strengths alkali-metal atoms elements group IIIA through second order many-body perturbation theor other correlated theories.

We report the result of our ab initio calculation $6{s}^{2}{S}_{1/2}\ensuremath{\rightarrow}5{d}^{2}{D}_{3/2}$ parity nonconserving electric dipole transition amplitude in $^{137}\mathrm{Ba}^{+}$ based on relativistic coupled-cluster theory. Considering single, double, and partial triple excitations, we have achieved an accuracy less than 1%. If can be matched by proposed nonconservation experiment ${\mathrm{Ba}}^{+}$ for above transition, then combination two results would provide...

Tests have been made to benchmark and assess the relative accuracies of low-order multireference perturbation theories as compared coupled cluster (CC) full configuration interaction (FCI) methods. Test calculations include ground some excited states Be, H2,BeH2,CH2, SiH2 systems. Comparisons with FCI CC show that in most cases effective valence shell Hamiltonian (Hv) method is more accurate than other theories, although none perturbative methods approximations. We also briefly discuss basic...

In view of the initial success complete active space (CAS) based size-extensive state-specific multireference perturbation theory (SS-MRPT) [J. Phys. Chem. A 103, 1822 (1999)] for relatively diverse yet simple chemically interesting systems, in this paper, we present computation potential energy curves (PEC) systems with arbitrary complexity and generality such as HF, H(2)O, H(2)S, C(2), N(2) molecules. The ground states (and also low-lying singlet excited C(2)) possess character making...

The state‐specific multireference perturbation theory ( SSMRPT ), which provides one state at a time may now gradually become new useful ab initio tool for studying electronic states with strong configurational quasidegeneracy owing primarily to its suitability toward numerical implementation in the presence of intruders and also great extent firm theoretical construct scope systematic hierarchical improvement. method works complete active space, treats each model space functions on same...

Adaptation of improved virtual orbitals (IVOs) in state-specific multireference perturbation theory using Møller-Plesset multipartitioning the Hamiltonian (IVO-SSMRPT) is examined which IVO-complete active space configuration interaction (CASCI) used as an inexpensive alternative to more involved CAS-self-consistent field (CASSCF) orbitals. Unlike CASSCF approach, IVO-CASCI does not bear tedious and costly iterations beyond those initial SCF calculation. The IVO-SSMRPT intruder-free,...

High-order multireference perturbation theory is applied to the $^{1}$S states of beryllium atom using a reference (model) space composed |1${\mathit{s}}^{2}$2${\mathit{s}}^{2}$〉 and |1${\mathit{s}}^{2}$2${\mathit{p}}^{2}$〉 configuration-state functions (CSF's), system that known yield divergent expansions Mo/ller-Plesset Epstein-Nesbet partitioning methods. Computations eigenvalues are made through 40th order forced degeneracy (FD) recently introduced optimization (OPT) partitioning. The...

Atomic properties involving the low-lying excited $^{2}D_{3∕2,5∕2}$ states in alkaline-earth-metal ions are of current interest many different applications ranging from tests physics beyond standard model to astrophysics. We have used relativistic coupled-cluster theory perform accurate calculations lifetimes lowest $^{2}D_{3∕2}$ and $^{2}D_{5∕2}$ singly ionized calcium, strontium, barium. The importance electron correlation these is emphasized. Our results compared with available...

The present work tests the suitability of narrow transitions $7s\phantom{\rule{0.3em}{0ex}}^{2}S_{1∕2}\ensuremath{\rightarrow}6d\phantom{\rule{0.3em}{0ex}}^{2}D_{3∕2}$ and $7s\phantom{\rule{0.3em}{0ex}}^{2}S_{1∕2}\ensuremath{\rightarrow}6d\phantom{\rule{0.3em}{0ex}}^{2}D_{5∕2}$ in ${\mathrm{Ra}}^{+}$ for optical frequency standard studies. Our calculations lifetimes metastable $6d$ states using relativistic coupled-cluster theory suggest that they are sufficiently long to be considered as a...

Multireference Møller-Plesset (MRMP) perturbation theory [K. Hirao, Chem. Phys. Lett. 190, 374 (1992)] is modified to use improved virtual orbitals (IVOs) and applied study ground state potential energy curves for isomerization dissociation of the N2H2 C2H4 molecules. In contrast traditional MRMP or multistate multiconfiguration quasidegenerate where reference functions are obtained from (often difficult converge) averaged self-consistent field methods, our represented in terms...

The relativistic coupled-cluster method is applied to calculate the magnetic dipole hyperfine constant $``A''$ of ${6s}_{1/2},$ ${6p}_{1/2},$ ${6p}_{3/2},$ and ${5d}_{3/2}$ states singly ionized barium. After inclusion two-body correlation effects into computation matrix elements, accuracy obtained values was significantly increased compared earlier computations. Based on these numbers calculations electric transitions excitation energies, an estimate for...

We present molecular applications of a spin free size-extensive state-specific multireference perturbation theory (SS-MRPT), which is valid for model functions arbitrary and generality. In addition to the singlet states, this method equally capable handle nonsinglet states. The formulation based on Rayleigh–Schrödinger approach works with complete active space treats each democratically. handling varying degrees quasidegeneracy ensuring size consistency as consequence extensivity. paper, we...

The improved virtual orbital (IVO) complete active space (CAS) configuration interaction (IVO-CASCI) method is a simplified CAS self-consistent field (SCF), CASSCF, method. Unlike the CASSCF approach, IVO-CASCI does not require iterations beyond an initial SCF calculation, rendering scheme computationally more tractable than and devoid of convergence problems that sometimes plague calculations as size increases, while retaining all essential positive benefits Earlier applications demonstrate...

The ionization potentials, transition energies, and oscillator strengths of Li Li-like C${}^{3+}$ Al${}^{10+}$ are computed at different plasma environments with the Fock-space multireference coupled-cluster theory to examine parametric dependence these properties on density and/or temperature. results presented here show that energies as well very sensitive environment. It further shows spectral lines corresponding $\ensuremath{\Delta}n=0$ transitions for blueshifted, whereas associated...

The correlated, size extensive ab initio effective valence shell Hamiltonian (HV) method is used to compute three-dimensional potential energy surfaces for the ground and several excited electronic states of H2S molecule. A single calculation HV simultaneously generates all interest as well ionization potentials. Particular emphasis placed on two lowest 1 1A″ (one valencelike other Rydberg-type) that are involved in recent experiments probing nonadiabatic photodissociation processes....

High order perturbation energies are computed for excited 1A1 states of BeH2 at geometries near the Be→H2 symmetric insertion transition state. The equations multireference theory solved through 30th to study difficulties in selecting appropriate zeroth Hamiltonian, orbitals, orbital energies, and reference functions computations smooth molecular potential energy surfaces. origin perturbative divergence produced by Möller–Plesset Epstein–Nesbet partitionings is analyzed using a conceptually...

We present our calculations of the electric dipole and quadrupole matrix elements for transitions between low-lying bound states ${\mathrm{Ba}}^{+}$ that are relevant parity nonconservation studies using relativistic coupled-cluster method. The results compare well with experimental data. have also computed transition many high-lying excited states.

Relativistic coupled-cluster(RCC) theory has been employed to calculate the life time of $6 ^2P_{3/2}$ state single ionized lead($^{207}Pb$) an accurac y 3% and compared with corresponding value obtained using second order r elativistic many-body perturbation theory(RMBPT). This is one very few ap plications this excited properties heavy atomic systems. C ontributions from different electron correlation effects are given explicitl y.

The electric quadrupole moment for the $4d\text{ }^{2}D_{5/2}$ state of $^{88}\mathrm{Sr}^{+}$; one most important candidates an optical clock, has been calculated using relativistic coupled-cluster theory. This is first application this theory to determine atomic moments. result calculation presented and many-body contributions are highlighted. $(2.94\ifmmode\pm\else\textpm\fi{}0.07)e{a}_{0}^{2}$, where ${a}_{0}$ Bohr radius $e$ electronic charge while measured value...

Effective valence shell Hamiltonian Hv calculations are used to map out three-dimensional potential energy surfaces for the 12 lowest electronic states of CaOH radical. Excitation energies and spectroscopic constants compared with experiment prior computations where available, but many previously unavailable data provided, including excited state dipole moments oscillator strengths. Particular attention is paid clarify nature nonlinear quasilinear states, Renner-Teller couplings, mixings....