The electron-attached (EA) and ionized (IP) symmetry-adapted-cluster configuration-interaction (SAC-CI) methods and their equation-of-motion coupled-cluster (EOMCC) analogs provide an elegant framework for studying open-shell systems. As shown in this study, these schemes require the presence of higher-order excitations, such as the four-particle-three-hole (4p-3h) or four-hole–three-particle (4h-3p) terms, in the electron attaching or ionizing operator R in order to produce accurate ground- and excited-state potential energy surfaces of radicals along bond breaking coordinates. The full inclusion of the 4p-3h/4h-3p excitations in the EA/IP SAC-CI and EOMCC methods leads to schemes which are far too expensive for calculations involving larger radicals and realistic basis sets. In order to reduce the large costs of such schemes without sacrificing accuracy, the active-space EA/IP EOMCC methodology [ J. R. Gour et al., J. Chem. Phys. 123, 134113 (2005) ] is extended to the EA/IP SAC-CI approaches with 4p-3h/4h-3p excitations. The resulting methods, which use a physically motivated set of active orbitals to pick out the most important 3p-2h/3h-2p and 4p-3h/4h-3p excitations, represent practical computational approaches for high-accuracy calculations of potential energy surfaces of radicals. To illustrate the potential offered by the active-space EA/IP SAC-CI approaches with up to 4p-3h/4h-3p excitations, the results of benchmark calculations for the potential energy surfaces of the low-lying doublet states of CH and OH are presented and compared with other SAC-CI and EOMCC methods, and full CI results.

10aconfiguration interactions10acoupled cluster calculations10aelectron attachment10aexcited states10afree radicals10aground states10aionisation10aorganic compounds10aoxygen compounds10apotential energy surfaces1 aOhtsuka, Y.1 aPiecuch, Piotr1 aGour, J., R1 aEhara, M.1 aNakatsuji, H. uhttps://icer.msu.edu/active-space-symmetry-adapted-cluster-configuration-interaction-and-equation-motion-cluster-methods01911nas a2200193 4500008004100000245014700041210006900188260001200257300001300269490000800282520117900290653003301469653001801502653002201520653002201542100001601564700001901580856011801599 2006 eng d00aEfficient Formulation and Computer Implementation of the Active-Space Electron-Attached and Ionized Equation-of-Motion Coupled-Cluster Methods0 aEfficient Formulation and Computer Implementation of the ActiveS c12/2006 a17 pages0 v1253 aThe efficient, general-purpose implementations of the active-space electron-attached (EA) and ionized (IP) equation-of-motion coupled-cluster (EOMCC) methods including up to 3p-2h and 3h-2p excitations, called EA-EOMCCSDt and IP-EOMCCSDt, respectively, are discussed. The details of the algorithm that enables one to achieve a high degree of code vectorization for the active-space methods and the factorized forms of the EA- and IP-EOMCCSDt equations that maximize the benefits of using active orbitals in the process of selecting the dominant 3p-2h and 3h-2p excitations are presented. The results of benchmark calculations for the low-lying doublet and quartet states of the CH and SH radicals reveal that the active-space EA-EOMCCSDt and IP-EOMCCSDt methods are capable of producing results for the electronic excitations in open-shell systems that match the high accuracy of EA- and IP-EOMCC calculations with a full treatment of 3p-2h and 3h-2p excitations, even when the excited states of interest display a manifestly multideterminantal nature, with the costs that can be on the same order of those characterizing the basic EOMCC singles and doubles approach.

10acoupled cluster calculations10afree radicals10aorganic compounds10asulphur compounds1 aGour, J., R1 aPiecuch, Piotr uhttps://icer.msu.edu/efficient-formulation-and-computer-implementation-active-space-electron-attached-and-ionized02915nas a2200313 4500008004100000245011100041210006900152260001200221300001600233490000800249520189000257653002102147653003102168653003302199653001302232653001902245653001302264653001802277653002102295653002302316653001802339653002202357653002202379653002902401653001002430100001902440700001702459856012502476 2005 eng d00aRenormalized coupled-cluster methods exploiting left eigenstates of the similarity-transformed Hamiltonian0 aRenormalized coupledcluster methods exploiting left eigenstates c12/2005 a224105–100 v1233 aCompletely renormalized (CR) coupled-cluster (CC) approaches, such as CR-CCSD(T), in which one corrects the standard CC singles and doubles (CCSD) energy for the effects of triply (T) and other higher-than-doubly excited clusters [ K. Kowalski and P. Piecuch, J. Chem. Phys. 113, 18 (2000) ], are reformulated in terms of the left eigenstates 〈Φ∣L of the similarity-transformed Hamiltonian of CC theory. The resulting CR-CCSD(T)L or CR-CC(2,3) and other CR-CCL methods are derived from the new biorthogonal form of the method of moments of CC equations (MMCC) in which, in analogy to the original MMCC theory, one focuses on the noniterative corrections to standard CC energies that recover the exact, full configuration-interaction energies. One of the advantages of the biorthogonal MMCC theory, which will be further analyzed and extended to excited states in a separate paper, is a rigorous size extensivity of the basic ground-state CR-CCL approximations that result from it, which was slightly violated by the original CR-CCSD(T) and CR-CCSD(TQ) approaches. This includes the CR-CCSD(T)L or CR-CC(2,3) method discussed in this paper, in which one corrects the CCSD energy by the relatively inexpensive noniterative correction due to triples. Test calculations for bond breaking in HF, F2, and H2O indicate that the noniterative CR-CCSD(T)L or CR-CC(2,3) approximation is very competitive with the standard CCSD(T) theory for nondegenerate closed-shell states, while being practically as accurate as the full CC approach with singles, doubles, and triples in the bond-breaking region. Calculations of the activation enthalpy for the thermal isomerizations of cyclopropane involving the trimethylene biradical as a transition state show that the noniterative CR-CCSD(T)L approximation is capable of providing activation enthalpies which perfectly agree with experiment.

10abonds (chemical)10aconfiguration interactions10acoupled cluster calculations10aenthalpy10aexcited states10afluorine10aground states10aheat of reaction10ahydrogen compounds10aisomerisation10amethod of moments10aorganic compounds10areaction kinetics theory10awater1 aPiecuch, Piotr1 aWloch, Marta uhttps://icer.msu.edu/renormalized-coupled-cluster-methods-exploiting-left-eigenstates-similarity-transformed-hamiltonian