Coupled-cluster theory, electrical properties

Haskopoulos and MarouUs [10] studied the interaction electric properties of H20 Rg (Rg = He, Ne, Ar, Kr, Xe). Correlation effects have been taken into account by employing M0Uer-Plesset (MP2, MP4) and coupled-cluster theories (CCSD, CCSD(T)) in connection with flexible, carefully designed basis sets. Bara-nowska et al. [11] computed the interaction-induced axial static dipole moments, polarizabilities and first hyperpolarizabilities of HCHO (HF)n (n= 1,2). They employed a series of methods (e.g. MP2, CCSD(T)) in connection with various basis sets. 

Abstract The modified equation-of-motion coupled cluster approach of Sekino and Bartlett is extended to computations of the mixed electric-dipole/magnetic-dipole polarizability tensor associated with optical rotation in chiral systems. The approach - referred to here as a linearized equation-of-motion coupled cluster (EOM-CCl) method - is a compromise between the standard EOM method and its linear response counterpart, which avoids the evaluation of computationally expensive terms that are quadratic in the field-perturbed wave functions, but still yields properties that are size-extensive/intensive. Benchmark computations on five representative chiral molecules, including (P)-hydrogen peroxide, (5)-methyloxirane, (5 )-2-chloropropioniuile, (/ )-epichlorohydrin, and (75,45)-norbornenone, demonstrate typically small deviations between the EOM-CCl results and those from coupled cluster linear response theory, and no variation in the signs of the predicted rotations. In addition, the EOM-CCl approach is found to reduce the overall computing time for multi-wavelength-specific rotation computations by up to 34%. 

Care must be taken in using the expressions above for obtaining nonlinear optical properties, because the values obtained may not be the same as those obtained from Eq. [4]. The results will be equivalent only if the Hellmann-Feyn-man theorem is satisfied. For the case of the exact wavefunction or any fully variational approximation, the Hellmann-Feynman theorem equates derivatives of the energy to expectation values of derivatives of the Hamiltonian for a given parameter. If we consider the parameter to be the external electric field, F, then this gives dE/dP = dH/d ) = (p,). For nonvariational methods, such as perturbation theory or coupled cluster methods, additional terms must be considered. 

Among the methods which guarantee adequate inclusion of the electron correlation effects, the coupled cluster (CC) theory is one of the most effective [4, 5, 33, 39, 59, 65, 79]. The first CC calculations of molecular properties related to the interaction of the molecule with the electric field date back to the first years of using the CC theory in molecular calculations [6, 14], However, the CC calculations, even those performed with the standard approach which includes single and double excitations from the reference wave function (CCSD), involve significant computational cost. 

Maroulis calculated the interaction-induced dipole polarizability and hyperpolarizability of the He2, Ne2, Ar2 and Kr2 homodiatoms relying on finite-field Moller-Plesset perturbation theory and coupled cluster calculations. Special attention was paid to the design of flexible basis sets, suitable for interaction-induced electric property calculations. Atom-specific, prepared basis sets were used on all atoms. The construction is completed in four steps ... 

Maroulis and Haskopoulos calculated the interaction electric dipole moment and polarizability for the C02-Rg systems, Rg = He, Ne, Ar, Kr and Xe. The potential minimum is very well defined for all these systems. In Fig. 19 is shown the potential energy surface for the C02-He interaction calculated at the MP2 level of theory. The most stable configuration corresponds to a T-shaped structure. The two local minima for the linear configuration of C02-He are also clearly visible. All interaction induced properties were extracted from finite-filed Moller-Plesset perturbation theory and coupled-cluster calculations with purpose-oriented basis sets. CCSD(T) values were calculated for the dipole moment pim of C02-He and C02-Ne the corresponding results are 0.0063 and 0.0107 eao, respectively. All post-Hartree-Fock methods yield stable values for this important property. For C02-He, = 0.0070 (SCF), 0.0063 (MP2), 0.0063 (MP4),... 

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