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The ground state geometries and associated normal mode frequencies of the classical and nonclassical protonated acetylene ion, i.e., the vinyl cation C2H3+, are computed using the complete active space self-consistent field and improved virtual orbital (IVO) complete active space configuration interaction methods. In addition, the minimum-energy reaction path for the classical to nonclassical interconversion is determined (as are excitation energies) using the IVO modification of multireference Mo\ller–Plesset (MRMP) perturbation theory. The IVO-MRMP treatment predicts the nonclassical structure to be 4.8 kcal/mol more stable than the classical one, which is consistent with other high level theoretical estimates. The proton affinity of acetylene from the IVO-MRMP treatment (154.8 kcal/mol) also agrees well with experiment (153.3 kcal/mol) and with earlier CASPT2 calculations (154.8 kcal/mol). We further report geometries and vibrational frequencies of low lying excited states of C2H3+, which have not been observed and/or studied before. Comparisons with previous highly correlated calculations further demonstrate the computational efficiency of the IVO-MRPT methods. [ABSTRACT FROM AUTHOR]

In view of the initial success of the 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 the computation of the potential energy curves (PEC) of systems with arbitrary complexity and generality such as HF, H2O, H2S, C2, and N2 molecules. The ground states of such systems (and also low-lying singlet excited states of C2) possess multireference character making the description of the state difficult with single-reference (SR) methods. In this paper, we have considered the Mo\ller–Plesset (MP) partitioning scheme [SS-MRPT(MP)] method. The accuracy of energies generated via SS-MRPT(MP) method is tested through comparison with other available results. Comparison with FCI has also been provided wherever available. The accuracy of this method is also demonstrated through the calculations of NPE (nonparallelism error) and the computation of the spectroscopic constants of all the above mentioned systems. The quality of the computed spectroscopic constants is established through comparison with the corresponding experimental and FCI results. Our numerical investigations demonstrate that the SS-MRPT(MP) approach provides a balanced treatment of dynamical and non-dynamical correlations across the entire PECs of the systems considered. [ABSTRACT FROM AUTHOR]

Multireference Mo\ller–Plesset (MRMP) perturbation theory [K. Hirao, Chem. Phys. Lett. 190, 374 (1992)] is modified to use improved virtual orbitals (IVOs) and is applied to study ground state potential energy curves for isomerization and dissociation of the N2H2 and C2H4 molecules. In contrast to traditional MRMP or multistate multiconfiguration quasidegenerate perturbation theory where the reference functions are obtained from (often difficult to converge) state averaged multiconfiguration self-consistent field methods, our reference functions are represented in terms of computationally efficient IVOs. For convenience in comparisons with other methods, a first order complete active space configuration interaction (CASCI) calculation with the IVOs is followed by the use of the IVOs in MRMP to incorporate residual electron correlation effects. The potential energy curves calculated from the IVO-MRMP method are compared with computations using state-of-the-art coupled cluster singles and doubles (CCSD) methods and variants thereof to assess the efficacy of the IVO-MRMP scheme. The present study clearly demonstrates that unlike the CCSD and its variants, the IVO-MRMP approach provides smooth and reliable ground state potential energy curves for isomerization of these systems. Although the rigorously size-extensive completely renormalized CC theory with noniterative triples corrections (CR-CC(2,3)) likewise provides relatively smooth curves, the CR-CC(2,3) calculations overestimate the cis-trans barrier height for N2H2. The ground state spectroscopic constants predicted by the IVO-CASCI method agree well with experiment and with other highly correlated ab initio methods. [ABSTRACT FROM AUTHOR]

The improved virtual orbital-complete active space configuration interaction (IVO-CASCI) method is extended to enable geometry optimization and the calculation of vibrational frequencies for ground and excited states using numerical energy gradients. Applications consider the ground state geometries and vibrational frequencies of the Be2, LiF, H2S, and HCN molecules, as well as excited state properties for HCN, systems that are sufficiently complex to access the efficacy of the method. Comparisons with other standard approaches (self-consistent field, second order Möller-Plesset perturbation theory, complete active space self-consistent field, and coupled cluster singles and doubles methods) demonstrate that the numerical gradient version of the IVO-CASCI approach generally fares comparable to or better for all systems studied. The accurate estimates for the Be2 bond length and vibrational frequency are notable since many other computationally facile methods produce poor results. [ABSTRACT FROM AUTHOR]

The relativistic effective valence shell Hamiltonian Hv 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 τ of excited states are evaluated and they are also in accord with experiment. Some of the calculated quantities have not previously been computed. [ABSTRACT FROM AUTHOR]

In the flash-photolysis of oxazole, iso-oxazole, and thiozole a transient band system was observed in the region 2500–3050 Å. This band system was attributed to a meta-stable form of HCN, i.e., either HNC or triplet HCN. Theoretical investigations have been carried out on the ground and excited states of HCN to characterize this and other experimentally observed transitions. The predicted geometries are compared with the experiment and earlier theoretical calculations. The present calculations show that the band system in the region 2500–3050 Å corresponds to the transition 4 3-A′←1 3-A′ of HCN. [ABSTRACT FROM AUTHOR]

Calculations of the ground and excited state potential energy curves of C2 using the third-order effective valence Hamiltonian (H3rdv) method are benchmarked against full configuration interaction and other correlated single-reference perturbative and nonperturbative theories. The large nonparallelity errors (NPEs) exhibited even by state-of-art coupled cluster calculations through perturbative triples indicate a serious deficiency of these single-reference theories. The Hv method, on the other hand, produces a much reduced NPE, rendering it a viable approximate many-body method for accurately determining global ground and excited state potential energy curves/surfaces. [ABSTRACT FROM AUTHOR]

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

Effective valence shell Hamiltonian Hv calculations are used to map out three-dimensional potential energy surfaces for the 12 lowest electronic states of the CaOH radical. Excitation energies and spectroscopic constants are compared with experiment and prior computations where available, but many previously unavailable data are provided, including excited state dipole moments and oscillator strengths. Particular attention is paid to clarify the nature of nonlinear and quasilinear excited states, Renner-Teller couplings, and state mixings. The F 2Π and G 2Π (6 2A′ and 8 2A′) states are both found to possess nonlinear local minima, due to an avoided crossing. Attention is also focused on the characteristics of basis sets necessary in high-accuracy calculations for the CaOH radical.© 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ionization potentials and excitation energies of rubidium and cesium atoms are computed using the relativistic coupled cluster (CC) method. The effect of electron correlations on the ground and excited state properties is investigated using different levels of CC approximations and truncation schemes. The present work demonstrates that the even-parity channel truncation scheme produces results almost as accurate as obtained from the all-parity channel approximation scheme at a reduced computational cost. The present study also indicates that for a given basis the linearized CC method tends to overestimate the ground and excited state properties compared to the full CC method. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Multireference many-body perturbative schemes (IVO–CASCI and H[sub v][sup 3rd]), which are applicable to the direct calculation of excitation energies, ionization potentials, and spectroscopic properties, are presented and applied to compute the transition energies, ionization potentials, and spectroscopic constants of TiN and VN. Highly satisfactory results are obtained for the excitation energies, triple bond dissociation energy, dipole moments, oscillator strengths, and vibrational frequencies. The ground and excited properties of interest are also computed using Hartree–Fock and two-component Dirac–Hartree–Fock molecular orbitals to assess the importance of relativistic effects. We also report the ionization potentials of TiN[sup +] and VN[sup +] which are by-products of this method with “no extra” computational cost and which have not been studied previously. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have applied the highly correlated ab initio effective valence shell Hamiltonian (H[sup v]) method to determine the energy difference between the cyclic and linear isomers of propynlidyne (C[sub 3]H). Calculations are also described for the vertical excitation energies, ionization potentials, electron affinities, dipole moments, oscillator strengths, and some harmonic vibrational frequencies, which are all determined using the third order H[sup v] method. Computations at both the experimental and theoretically optimized geometries are used to illustrate the geometrical dependence of the computed properties. The H[sup v] optimized geometry is obtained using a two-configurational reference function describing the two dominant resonance structures. Our third-order vertical excitation energy to the lowest excited state in the cyclic isomer, dipole moments, and ground state isomer conformational energy difference are all in good agreement with experiment and with other highly correlated many-body calculations. The computations for higher excited states and for ionization potentials, electron affinities, and oscillator strengths represent the first reports of these quantities. An explanation is provided for persistent theoretical difficulties in computing b[sub 1] bending vibrational frequencies of the cyclic isomer. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The adiabatic energies and geometries of the ground and excited electronic states of formylcarbene (HC.CHO) and the triplet electronic states of ketene ([H.sub.2]CCO) are studied. The employed methods are shown to be extremely effective and accurate.

Derives the analytic energy gradients and nondiabatic derivative couplings to evaluate the derivatives of the molecular orbitals. Simultaneous treatment of energy derivatives and nondiabatic couplings for several states; Modification of the general coupled perturbed method; Development of analytical energy derivative.

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 the H2S molecule. A single calculation of the HV simultaneously generates all states of interest as well as ionization potentials. Particular emphasis is placed on the two lowest 1 1A″ excited surfaces (one valencelike and the other Rydberg-type) that are involved in recent experiments probing nonadiabatic photodissociation processes. Supplementary effective operator calculations generate three-dimensional surfaces of dipole moments and transition dipole matrix elements, but emphasis is placed on the transition dipoles relevant to the dissociation process. Comparisons to both experiment and previous calculations for this system support the ability of multireference perturbation methods to describe global potential energy surfaces for open shell systems. We discuss the implication of our calculations for interpreting and reproducing experimental observations of the dissociation dynamics. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The minimum basis set hydrogen rectangular system (HRS), consisting of four hydrogen atoms arranged in a rectangle, is examined using a variety of partitionings of the Hamiltonian H for high order single and double reference perturbation computations. The potential energy surface is mapped out over a range of geometries in which the length L of one side of the rectangle is varied. Several criteria are derived governing the necessary conditions for perturbative convergence of two-state systems, and these criteria are useful in explaining the behavior of the HRS for the range of geometries and partitioning methods investigated. The divergence caused by intruder states, observed by Zarrabian and Paldus [Int. J Quantum Chem. 38, 761 (1990)] for the nondegenerate, double reference space perturbation expansions at L=3.0 a.u. with traditional partitioning methods, is shown to correspond to avoided crossings with negative real values of the perturbation parameter—backdoor intruder states. These intruder state induced divergences result from too small zeroth order energy differences between the high lying reference space state and an orthogonal space intruder state whose identity depends on the partitioning method. Forcing the valence orbitals to be degenerate enlarges these zeroth order energy differences and, thus, yields a convergent perturbative expansion for L=3.0 a.u.The convergent or divergent behavior of all the partitioning method computations and the locations of their avoided crossings are accurately predicted by using two-state models composed of the high lying reference space state and the intruder state. A partitioning method is introduced in which the zeroth order state energies are selected to optimize the convergence in low orders of the perturbation expansion. This optimization method yields perturbative convergence which is both rapid and free of intruder state for geometries between L=2.0 and 3.0 a.u. The divergent behavior for various... [ABSTRACT FROM AUTHOR]

Examines the application of valence shell Hamiltonian method to estimate the excitation energies and oscillator strengths of magnesium-like ions. Significance of size extensibility for potential energy surfaces; Relationship between basis set size and polarization; Dependence of oscillator strengths on the nature and choice of valence orbitals.

Analyzes the efficacy of various multireference perturbation methods (MRPT) using high order perturbative computations for the lowest lying single states of the CH[sub 2] molecule. Comparison of the methods; Necessary ingredients for useful MRPT methods.

Discusses the use of the ab initio effective valence shell Hamiltonian (H [sup v] to compute the low lying vertical excitation energies and oscillator strengths for ethylene, trans-butadiene, benzene and cyclobutadiene. Agreement of calculated excitation energies with experiment and with valued from other highly correlated computations.

Discusses the high order perturbation energies for excited states of beryllium hydride at geometries near the symmetric insertion transition state. Solution of equations of multireference perturbation theory through 30th order; Origin of perturbative divergence produced by Moler-Plesset and Epstein-Nesbet partitionings.

The equation of motion coupled cluster methodology within relativistic framework has been applied to analyze the electron correlation effects on the low lying dipole allowed excited states of Ne and Al3+ under classical and quantum plasma environments. The effect of confinement due to classical plasma has been incorporated through screened Coulomb potential, while that of quantum plasma has been treated by exponential cosine screened Coulomb potential. The confined structural properties investigated are the depression of ionization potential, low lying excitation energies (dipole allowed), oscillator strengths, transition probabilities, and frequency dependent polarizabilities under systematic variation of the plasma-atom coupling strength determined through the screening parameter. Specific atomic systems are chosen for their astrophysical importance and availability of experimental data related to laboratory plasma with special reference to Al3+ ion. Here, we consider 1 s 2 2 s 2 2 p 6 ( 1 S 0 ) → 1 s 2 2 s 2 2 p 5 n s / n d ( 1 P 1 ) ( n = 3 , 4 ) dipole allowed transitions of Ne and Al3+. Results for the free (isolated) atomic systems agree well with those available in the literature. Spectroscopic properties under confinement show systematic and interesting pattern with respect to plasma screening parameter. [ABSTRACT FROM AUTHOR]

To obtain even qualitatively correct results of potential energy surfaces (PESs) of the ground and two lowest-lying excited singlet states of C2is a difficult task due to the strong geometry-dependent closeness of these PESs leading to real and avoided crossings. Our IVO-SSMRPT method in which the nondynamical correlation is simplified with IVO-CASCI has been applied to these states. IVO-SSMRPT provides comparable results with reference theoretical and experimental data indicating all components of the approach work in harmony for a correct representation of the surfaces including the locations of the challenging crossing points between the states. [ABSTRACT FROM PUBLISHER]