Application of multireference linearized coupled-cluster theory to atomic and molecular systems

J. Chem. Theory Comput.

2022

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Block effective Hamiltonian theory (BEHT) is presented in this work. Configuration interaction functions are divided into P, Q, and R spaces. Effective Hamiltonian is constructed with the partitioning technique within the P space. The eigenvalue problem of the effective Hamiltonian is then solved iteratively. It is demonstrated that the ground-state energies of N 2 , HF, and F 2 calculated with BEHT converge to the multireference configuration interaction energies from below and the iteration number becomes smaller as BEHT energy becomes closer to the exact energy. The accuracy of BEHT is better than that of the second-order multireference perturbation theory, although the matrix elements in both methods are the same. The ionization potentials of the singlet state of HF, the doublet state of F, and the triplet state of NH and the potential energy curves of CH 4 , C 2 , and N 2 are calculated with BEHT and compared with experiments and results of CASSCF, CCSD, and CCSD(T) and the results of the full configuration interaction if available. The iteration numbers are all less than 10 in this study. These calculations show the good performances of BEHT in comparison with other theoretical approximation methods.

Section: Resultssupporting

confidence: 82%

“…This was observed in our previous studies on electron affinity. 33,69 The orbitals of F and F + are CASSCF (5,7) and CASSCF(4,7), respectively. The corresponding P spaces of F and F + are, respectively, CAS(5,7) and CAS (4,7).…”

Section: Ips Of Hf F and Nhmentioning

confidence: 99%

Block Effective Hamiltonian Theory and Its Application

Hou

J. Chem. Theory Comput.

2022

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Multireference Rayleigh–Schrödinger perturbation theory and its application

Jiang

The Journal of Chemical Physics

2019

Based on the complete active space multireference wavefunction, multireference Rayleigh–Schrödinger perturbation theory (MRSPT) is derived with the assumption that the orbital energies of active orbitals are the same as ε¯, an unknown parameter. In this work, ε¯ is optimized at the MRSPT2 level. The second and third order perturbation theories are shown numerically to be size extensive. The second order perturbation theory is exploited to compute the ground state energies of F2, AlH, HCl, and P2 and to optimize the equilibrium bond lengths and harmonic vibrational frequencies of BH, BF, P2, HF, and F2. The dissociation behaviors of NH3 and OH− have also been investigated. Comparisons with other theoretical models as well as the experimental data have been made to show advantages of the present theory.

Theoretical study on divergence problems of single reference perturbation theories

Jiang

Xie

Chinese Journal of Chemical Physics

2019

Divergences of the single reference perturbation theories due to the addition of diffusion basis functions have been investigated for both closed-shell and open-shell molecular systems. It is found that the oscillatory range of perturbation energies of open-shell systems increases as the spin multiplicity of systems changes from 2 to 4. Feenberg transformation is exploited to treat the divergence problems. It is found numerically that within the interval of Feenberg parameter there exists a minimum perturbation order at which the perturbation series become convergent. It is also found for the open-shell systems that the magnitude of the corresponding Feenberg parameter becomes larger as the spin multiplicity of the system of interest changes from 2 to 4.