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Mitarai, K. Fujii, Phys. Rowe, Rev. Ollitrault, A. Kandala, C.. Chen, P. Barkoutsos, A. Mezzacapo, M. Pistoia, S. Sheldon, S. Woerner, J. Gambetta, I. Dewes, R. Lauro, F. Ong, V. Schmitt, P.
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Bushnell, Y. Chen, Z. Chen, B. Chiaro, R. Collins, W. Courtney, S. Demura, A. Dunsworth, E. Farhi, A. Fowler, B. Foxen, C. Gidney, M. Giustina, R. Graff, S. Habegger, M. Harrigan, A.
Ho, S. Hong, T. Huang, W. Huggins, L. Ioffe, S. Isakov, E. Jeffrey, Z. Jiang, C. Jones, D. Kafri, K. Kechedzhi, J. Kelly, S. Kim, P. Klimov, A. Korotkov, F. Kostritsa, D. Landhuis, P. Laptev, M. Lindmark, E. Lucero, O. Martin, J. Martinis, J. McEwen, A. Megrant, X. Mi, M. Mohseni, W. Mruczkiewicz, J. Mutus, O. Naaman, M. Neeley, C. Neill, H. Neven, M. Niu, T. Ostby, A. Petukhov, H.
Putterman, C. Quintana, P. Roushan, N. Rubin, D. Sank, K. Satzinger, V. Smelyanskiy, D. Strain, K. Sung, M. Valence-universal or Fock space: Suitable for difference energies Mukherjee, Kutzelnigg, Lindgren, Kaldor and others Common vacuum concept; Wave-operator consists of hole- particle creation, but also destruction of active holes and particles contained in the model space.
State-universal or Hilbert space: Suitable for the potential energy surface. To make the theory applicable to energy derivatives like properties or gradients, Hessians etc. Pal, Phys. Rev A 39, 39, ; S. Pal, Int. N-electron RHF chosen as a vacuum, with respect to which holes and particles are defined.
Heff is the effective Hamiltonian defined over the model space determinants. The eigen values of it gives the exact energies of interest. For low-lying excited states one hole one particle model space is suitable.
Though generally for IMS, to have linked cluster theorem, intermediate normalization has to be abandoned, for 1,1 model space, the equations can be derived assuming intermediate normalization. Computationally full singles and doubles approximation has been used. For excitation energies closed part of the connected H exp T 0,0 is dropped, to facilitate direct evaluation.
Ajitha, N. Vaval and S. Pal, J Chem Phys , ; J. Phys , ;. Z - vector although perturbation independent, still depends on state of interest No single Z- vector for all roots at the same time. K R Shamasundar and S. Pal, J. Specific expressions derived for [0,1], [1,0] and [1,1] sectors and first order response of energy calculated. Factorization of response equation possible only for T[0,1] 1 and is only for the highest valence case. Elimination of T 1 can be carried out separately for each element E 1.
Shamasundar and Pal, Int. J Mol. The stationary equations are obtained by making the Lagrange functional stationary with respect to the T amplitudes, amplitudes and effective Hamiltonian elements. Hence in the stationary equation with respect to T[i] summation index is from i to n , where n is the highest valence sector.
To solve Lagrange multipliers for a specific sector, all higher valence s are necessary. Rigorous spectra and properties Accurate calculation of difference energies using multi-reference coupled-cluster method accuracy within 0. Pal, M. Rittby , R.
Bartlett, D. Sinha and D. Mukherjee J. Vaval, K. Ghose, S. Pal and D. Mukherjee, Chem. Phys , ; S. Pal, Theor. A, 33, ; Phys. A,39,39, ; N. Ghose and S. Phys, , , ; N. A 54, ; D. Manohar and S. Pal, Chem. Anal a. Manohar, Vaval and Pal , Theo.
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The basis for CI methods is the simple observation that an exact many-body wavefunction, , may be written as a linear combination of Slater determinants, , 2. In most electronic systems, the Hartree-Fock energy accounts for the majority of the exact total energy, and the missing correlation energy is small.
If the coefficients are normalised then typically and all remaining are very small. A very large number of configurations is required to yield energies and wavefunctions approaching the exact many-body wavefunction.
In practice the expansion must be limited on physical grounds, as the total number of determinants is 2.
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