S. Ajith Perera

Electron correlation effects on the theoretical calculation of nuclear magnetic resonance spin–spin coupling constants

The equation‐of‐motion coupled cluster singles and doubles (EOM‐CCSD) method for general second‐order properties is derived providing a quadratic, CI‐like approximation and its linked form from coupled cluster (CC) energy derivative theory. The effects of the quadratic contribution, of the atomic basis set employed, and of electron correlation on NMR spin–spin coupling constant calculations using EOM‐CCSD methods are investigated for a selected set of difficult molecules, notably CH3F, B2H6, CH3CN, C2H4, and CH3NH2. We find that the quadratic contribution is insignificant for the couplings in the molecules considered in this study and in addition the quadratic contribution only slightly depends on the basis set used. Therefore it seems well justified to use the less expensive CI‐like approximation or its linked‐diagram form to evaluate spin–spin coupling constants. The Fermi‐contact contribution shows the largest variation with the change of basis sets. The diamagnetic spin–orbit (DSO) and the spin–dipole...

COUPLED-CLUSTER CALCULATIONS OF INDIRECT NUCLEAR COUPLING CONSTANTS : THE IMPORTANCE OF NON-FERMI CONTACT CONTRIBUTIONS

Electron correlation effects to the four coupling mechanisms which contribute to the isotropic nuclear spin–spin coupling constant, the Fermi contact (FC), paramagnetic spin–orbit (PSO), spin‐dipole (SD), and diamagnetic spin–orbit (DSO) are studied using the equation of motion coupled‐cluster (EOM‐CC) method. The second‐order properties are expressed as a sum‐over state (SOS) using EOM‐CC intermediate state wave functions. This formulation is simple, accurate, computationally convenient, and involves no truncation. Several molecules, HF, CO, N2, H2O, NH3, and HCl which have been previously shown to have large noncontact contributions are investigated using the EOM‐CC method and the results are compared with experiment and other theoretical methods, including polarization propagator and finite‐field MBPT(2) methods. Using fairly large basis sets, the EOM‐CCSD provides results which agree with experimental indirect nuclear spin–spin coupling constants to within an average error of 13%.

Critical comparison of single-reference and multireference coupled-cluster methods: Geometry, harmonic frequencies, and excitation energies of N2O2

Calculated vertical excitation energies, optimized geometries, and vibrational frequencies of the nitric oxide dimer are reported. The “multireference” (MR) nature of the problem and weak bond between the monomers make a proper description of the system difficult, and standard methods are not as applicable to this system. In this study, recently developed methods such as the double-electron-affinity similarity-transformed equation-of-motion coupled cluster method (DEA-STEOM-CCSD), MR Brillouin–Wigner CCSD (MR-BWCCSD), MR average quadratic CCSD (MR-AQCCSD), and others are used along with a series of basis sets of increasing accuracy. The calculated excitation energies are consistent and convergent with respect to the basis set in DEA-STEOM-CCSD, MR-BWCCSD, and MR-AQCCSD methods. The geometries are highly sensitive to the basis set size and the challenge to obtain the right answers in the basis set limit remains. Nevertheless, we obtain qualitative agreement with the experimental geometry and harmonic vibra...

A THEORETICAL STUDY OF HYPERFINE COUPLING CONSTANTS

Isotropic hyperfine coupling constants of first‐row atoms from B–F and the BH2 radical are calculated analytically from the coupled‐cluster (CC) relaxed density with a variety of extended basis sets. We employ both restricted and unrestricted Hartree–Fock reference functions, with the CC singles and doubles (CCSD), CCSD with noniterative triples [CCSD+T(CCSD) and CCSD(T)] methods. The latter provide excellent agreement with experiment. We also consider the role of orbital relaxation and atomic basis functions in accurate predictions.

Coupled-cluster calculations of Raman intensities and their application to N4 and N5−

Abstract Raman intensities are evaluated by numerically differentiating the analytically calculated coupled-cluster singles and doubles (CCSD) polarizabilities with respect to small geometric perturbations. The CCSD Raman intensities are calibrated by comparison to several molecules with experimentally known Raman Intensities. The CCSD Raman intensities agree well with experiment. Predictions for tetrahedral N 4 , and the pentazole anion N 5 − are reported.

Correlated calculations of molecular dynamic polarizabilities

Frequency-dependent molecular polarizabilities of several molecules N2, CO, CO2, Cl2, C2H2, COS, and CS2 are calculated by the equation-of-motion coupled cluster singles and doubles (EOM-CCSD) method. The EOM-CCSD CI-like, linear and quadratic methods for dynamic second-order properties are presented. The importance of electron correlation, the quadratic contribution, and orbital relaxation effects are assessed. London dispersion coefficients are calculated by numerical integration of the EOM-CCSD polarizabilities.

Relativistic effects at the correlated level. An application to interhalogens

Abstract Relativistic effects are significantly important in understanding the electronic structure and spectra of heavy atoms or molecules containing such atoms. Estimation of relativistic effects at the coupled-cluster (CC) level, using the Cowan—Griffin quasi-relativistic many-electron Hamiltonian, is described. The relativistic effects are treated as an external perturbation to the non-relativistic Born—Oppenheimer many-electron Hamiltonian and are evaluated analytically by the “relaxed” density formulation of coupled-cluster (CC) theory. Applicability of the method is demonstrated by calculating relativistic corrections to the ground state energies and dipole moments of diatomic interhalogens. We also present comparisons with relativistic effective potential results.

HYPERFINE COUPLING CONSTANTS OF ORGANIC RADICALS

The isotropic hyperfine coupling constants of several organic radicals including CH3, CH2, CH2−, C2H5, C2H3, H2CN, C6H7, and C3H5 are calculated analytically using the coupled cluster (CC) “relaxed density’’ matrix approach. We employ three different commonly used basis sets with CCSD and CCSD(T) in order to calibrate expected accuracy. The Chipman basis set combined with the CCSD(T) method performs best for carbon isotropic hyperfine coupling constants with a mean absolute deviation within 8% compared to experiment. The corresponding mean absolute deviation for hydrogen isotropic hyperfine coupling constants from experiment is 12%. We show that the UHF, ROHF, and quasi (QRHF) reference function CCSD spin densities are effectively numerically equivalent in the notorious case of the allyl radical.

First calculations of 15N-15N J values and new calculations of chemical shifts for high nitrogen systems: a comment on the long search for HN5 and its pentazole anion.

In the potential solution observation of the long-sought-after pentazole anion (N(5)(-)), the principal experimental tool used for detection is NMR. However, in two experiments, very different conclusions were reached. To assist in the interpretation, we report predictive level coupled-cluster results for the spin-spin coupling constants and chemical shifts for all of the key species, which include NO(3)(-), N(5)(-), HN(5), N(3)(-), and MeOC(6)H(5)N(3). In the case of the shifts, an empirical estimate based on the molecule polarity enables comparison of gas-phase and observed values with expected error bars of approximately +/-10 ppm. For the scalar couplings, the evidence is that the solution effects are modest, enabling the gas-phase values (with error bars are approximately +/-5 Hz) to be accurate. The latter supports the observation of centrally (15)N labeled N(3)(-) in the cerium(IV) ammonium nitrate (CAN) solution which could only occur if the pentazole anion had been created in the experiment, yet with too short a lifetime to be observed in NMR.

Vertical ionization potentials of ethylene: the right answer for the right reason?

The photoelectron spectrum of ethylene is studied using coupled cluster methods, including an existing ambiguity in what are reported to be its experimental vertical ionization potentals. Two complementary methods are used for generating the ionization potentials: δE CCSD(T) and IP-EOM-CCSD. The adiabatic IP of the neutral molecule in the ground state is well known and widely accepted to be 10.5122eV. The basis set extrapolated adiabatic IPS with zero-point corrections are 10.46 eV and 10.56 eV, respectively, but a vibronic coupling between the ground state cation and its first excited state can reduce these values by ∼0.03 eV. From an exponential basis set extrapolation the vertical ionization potentials are predicted to be 10.8 eV (B3u, 13.2eV (B3g 15.0eV (Ag), 16.4eV (B2u), and 19.6eV (B1u) ±0.1 eV.