Nayana Vaval

Ab initio lifetimes in the interatomic Coulombic decay of neon clusters computed with propagators

Interatomic Coulombic decay (ICD) is a radiationless decay mechanism occurring via electron emission in an inner-valence ionized weakly bound cluster. The ICD has been studied for the neon clusters Nen (n=2,...,5). The decay widths of the neon clusters are calculated using ab initio Greens function method. The non-Dyson version of Greens function is employed. This propagator is analytically continued into the complex energy plane with the aid of a complex absorbing potential, and the decaying states are found as resonance states in this plane.

Multireference coupled cluster based analytic response approach for evaluating molecular properties: Some pilot results

In this paper we present the first results for linear response theory in the multireference Fock space framework. Dipole moments of the open shell radicals hydro peroxy radical, hydroxyl radical, and formyloxyl radical are discussed in this paper. The results obtained from the analytic response are compared with the experimental as well as the finite field results including relaxation of the orbitals and the results suggest that the effects of relaxation are substantial. The Fock space approach enables us to obtain the properties of excited states of these radicals in a single calculation. The dipole moments of the first excited states of the above radicals are also reported. The results of the lower Fock space sector energy derivatives are also presented.

Equation-of-motion coupled-cluster method for the study of shape resonance

The equation-of-motion coupled-cluster method (EOM-CC) is applied for the first time to calculate the energy and width of a shape resonance in an electron-molecule scattering. The procedure is based on inclusion of complex absorbing potential with EOM-CC theory. We have applied this method to investigate the shape resonance in e(-)N(2), e(-)CO, and e(-)C(2)H(2).

Nonlinear molecular properties using biorthogonal response approach

In this paper, we report the use of extended coupled cluster functional of Arponen, Bishop, and co‐workers to implement a stationary biorthogonal response approach. The objective of this is to calculate nonlinear molecular properties like hyperpolarizability, etc. in a more convenient way.

Performance of the EOMIP-CCSD(2) Method for Determining the Structure and Properties of Doublet Radicals: A Benchmark Investigation.

We present a benchmark study on the performance of the EOMIP-CCSD(2) method for computation of structure and properties of doublet radicals. The EOMIP-CCSD(2) method is a second-order approximation to the standard EOMIP-CCSD method. By retaining the black box nature of the standard EOMIP-CCSD method and adding favorable N(5) scaling, the EOMIP-CCSD(2) method can become the method of choice for predicting the structure and spectroscopic properties of large doublet radicals. The EOMIP-CCSD(2) method overcomes the typical problems associated with the standard single reference ab initio treatment of doublet radicals. We compare our results for geometries and harmonic vibrational frequencies with those obtained using the standard EOMIP-CCSD method, as well as unrestricted Hartree-Fock (UHF)- and restricted open-shell Hartree-Fock (ROHF)-based single-reference coupled-cluster and second order many-body perturbation theory (MBPT(2)) methods. The effect of the basis set on the quality of the results has been studied using a hierarchy of Dunnings correlation-consistent aug-cc-pVXZ (X = D, T, Q) basis sets. Numerical results show that the EOMIP-CCSD(2) method, despite its N(5) scaling, gives better agreement with experimental results, compared to the UHF- and ROHF-based MBPT(2), as well as the single-reference coupled-cluster methods.

Fock-space multireference coupled-cluster theory. Fourth-order corrections to the ionization potential

We propose a highly correlated scheme for implementing a Fock-space multireference coupled-cluster theory for computing directly ionization potentials (IPs). We include all the single and double excitation cluster amplitudes and certain contributions from the triples which contributed at third and fourth orders of perturbation. The computed IP values from our scheme are correct at least up to fourth order. Small numerical test cases are given as examples to illustrate the importance of the fourth-order triples.

First- and second-order electrical properties computed at the FSMRCCSD level for excited states of closed-shell molecules using the constrained-variational approach

Fock space multireference coupled-cluster (FSMRCC) method emerged as an efficient tool to describe the electronic structure of nearly degenerate cases. Development of linear response has been one of the challenging problems in FSMRCC due to the multiple-root nature of the effective Hamiltonian. A response from any of the roots would span the space for getting the properties. Hence, all roots perturbed by the external field would proliferate the excited states. We recently developed the FSMRCC method for the efficient evaluation of analytic response properties using a constrained variation approach. In this paper, we present analytic dipole moments and polarizabilities of H(2)O, O(3), and CH(+) molecules in low-lying excited states along with brief discussion of singlet triplet decoupling of (1,1) sector of FSMRCC resulting from spin adaptation.

Fock space multireference coupled cluster calculations based on an underlying bivariational self-consistent field on Auger and shape resonances.

The Fock space multireference coupled cluster based on an underlying bivariational self-consistent field is applied to the problem of computing complex energy associated with Auger and shape resonances in e-atom scattering. It is concluded that the Fock space multireference coupled cluster based on a bivariational self-consistent field provides a useful and practical approach to calculation of resonance parameters. Numerical results are presented for the 2P shape resonance of Mg and Auger 1 s(-1) hole of Be.

Partitioned EOMEA-MBPT(2): An Efficient N(5) Scaling Method for Calculation of Electron Affinities.

We present an N(5) scaling modification to the standard EOMEA-CCSD method, based on the matrix partitioning technique and perturbative approximations. The method has lower computational scaling and smaller storage requirements than the standard EOMEA-CCSD method and, therefore, can be used to calculate electron affinities of large molecules and clusters. The performance and capabilities of the new method have been benchmarked with the standard EOMEA-CCSD method, for a test set of 20 small molecules, and the average absolute deviation is only 0.03 eV. The method is further used to investigate electron affinities of DNA and RNA nucleobases, and the results are in excellent agreement with the experimental values.

Adiabatic states of ozone using Fock space multireference coupled cluster method

In this paper we present the Fock space multireference coupled cluster theory suitable for calculation of low-lying adiabatically excited or electron attached states. Low-lying adiabatic as well as vertical excited states of ozone are calculated using this theory in singles and doubles approximation. The calculated adiabatic excitation energies are compared with the experimental values. We also report the adiabatic electron affinity value of ozone.