Datta Dipayan, Gauss Jürgen
Institut für Physikalische Chemie, Johannes Gutenberg-Universität Mainz, Duesbergweg 10-14, D-55128 Mainz, Germany.
J Chem Phys. 2014 Sep 14;141(10):104102. doi: 10.1063/1.4894773.
An analytic scheme is presented for the evaluation of first derivatives of the energy for a unitary group based spin-adapted coupled cluster (CC) theory, namely, the combinatoric open-shell CC (COSCC) approach within the singles and doubles approximation. The widely used Lagrange multiplier approach is employed for the derivation of an analytical expression for the first derivative of the energy, which in combination with the well-established density-matrix formulation, is used for the computation of first-order electrical properties. Derivations of the spin-adapted lambda equations for determining the Lagrange multipliers and the expressions for the spin-free effective density matrices for the COSCC approach are presented. Orbital-relaxation effects due to the electric-field perturbation are treated via the Z-vector technique. We present calculations of the dipole moments for a number of doublet radicals in their ground states using restricted open-shell Hartree-Fock (ROHF) and quasi-restricted HF (QRHF) orbitals in order to demonstrate the applicability of our analytic scheme for computing energy derivatives. We also report calculations of the chlorine electric-field gradients and nuclear quadrupole-coupling constants for the CCl, CH2Cl, ClO2, and SiCl radicals.
本文提出了一种分析方案，用于评估基于酉群的自旋适配耦合簇（CC）理论中能量的一阶导数，即在单双激发近似下的组合开壳层CC（COSCC）方法。在推导能量一阶导数的解析表达式时，采用了广泛使用的拉格朗日乘数法，该方法与成熟的密度矩阵公式相结合，用于计算一阶电学性质。文中给出了用于确定拉格朗日乘数的自旋适配λ方程的推导，以及COSCC方法的无自旋有效密度矩阵的表达式。通过Z矢量技术处理了电场微扰引起的轨道弛豫效应。为了证明我们的解析方案在计算能量导数方面的适用性，我们使用受限开壳层哈特里-福克（ROHF）和准受限HF（QRHF）轨道，对一些基态双重自由基的偶极矩进行了计算。我们还报告了CCl、CH2Cl、ClO2和SiCl自由基的氯电场梯度和核四极耦合常数的计算结果。
