No abstract

Recently developed single-root multireference Brillouin–Wigner coupled-cluster (MR BWCC) theory, which belongs to a broad family of state-selective multireference coupled-cluster methods, has been implemented in the ACES II program package at the CCSD level of approximation. The method represents a new approach to quasidegenerate problems, which is able to continuously switch between the single-reference CC in a nondegenerate situation and the Hilbert-space MRCC in a degenerate case. An assessment of the method has been carried out by means of a comparison with the full configuration interaction (CI) treatments of CH2, SiH2, and twisted ethylene diradicals. The problem of size-extensivity is discussed.

We present a simple a posteriori correction for the state-specific multireference Brillouin–Wigner coupled-cluster (MR BWCCSD) theory, which eliminates its size-extensivity error. In the converged amplitudes we drop terms that were identified to be responsible for the lack of size extensivity. We performed MR BWCCSD calculations with this correction on CH2, SiH2, twisted ethylene, F2, and ozone that are all, from the computational point of view, typical representatives of two-reference problems. Comparison with rigorously size-extensive calculations and experiment shows that the size-extensivity error of the corrected MR BWCCSD is only a few tenths of kcal/mol.

No abstract

Ab initio MO and density functional calculations indicate that the ring opening of the cyclobutene radical cation (CB •+) follows two competitive pathways, whose energy barriers differ by less than 1 kcal/mol at the highest level of theory employed, RCCSD(T)/cc-pVTZ//UQCISD/6-31G*. The first corresponds to a conrotatory rearrangement to the cis-butadiene radical cation (cis-BD •+). The second one leads to trans-BD •+ via a very flat potential energy plateau which comprises cyclopropylcarbinyl-type structures of the type proposed some time ago by Bauld, but the controtatory stereochemistry is preserved also along this process. State correlation diagrams indicate that the rearrangement leading to trans-BD •+ may occur adiabatically along a C 2 reaction coordinate. Despite this, the transition state has no symmetry. This seemingly “unnecessary” loss of symmetry is traced back to the proximity of the 2A and 2B surfaces in the vicinity of the C 2 stationary points, where the two states encounter a strong vibronic interaction which leads to breaking of the C 2 symmetry. These vibronic interactions are also responsible for the general flattening of the potential energy surface in this area.

Automerization reaction of cyclobutadiene and its barrier height: An ab initio benchmark multireference averagequadratic coupled cluster study J. Chem. Phys. 125, 064310 (2006); 10.1063/1.2222366 Continuous transition between Brillouin-Wigner and Rayleigh-Schrödinger perturbation theory, generalized Bloch equation, and Hilbert space multireference coupled cluster Hilbert-space state-universal multireference coupled-cluster ͑MR CC͒ data on cyclobutadiene ͓A. Balková and R. J. Bartlett, J. Chem. Phys. 101, 8972 ͑1994͔͒ were used as a benchmark for testing our recently developed state-specific ͑single-root͒ multireference Brillouin-Wigner coupled-cluster ͑MR BWCC͒ theory. For the energy of activation in the automerization of cyclobutadiene ͑i.e., the energy difference between the square and rectangular structures͒ at the CCSD/͓3s2 p1d/2s͔ level of theory, our MR BWCCSD method gives the value of 6.2 kcal/mol, compared to 6.5 kcal/mol given by MR CCSD. With the cc-pVDZ and cc-pVTZ/cc-pVDZ basis sets, the MR BWCCSD activation barrier is 6.4 and 7.0 kcal/mol, respectively. The effect of the triple excitations ͓in MR CCSD͑T͔͒ and of the frozen core approximation were estimated previously to be below 0.1 kcal/mol and in the opposite direction. This shows the way of how to arrive at a more accurate automerization barrier in future calculations: extension of the basis set seems to be more important than going beyond the CCSD͑T͒ or MB BWCCSD level of the theory.

Propargylene was identified in a matrix as a product of photolysis of cyclopropenylidene and diazopropyne. The molecule is a triplet. The optimum geometry predicted by ab initio calculations corresponds to a structure HC≡C–C̈H. The transition structure in the interconversion HC≡C–C̈H⇄HĊ=C=ĊH⇄HC̈–C≡CH is very low in energy and close to the energy of the vibrational ground state. Owing to this nonrigidity, computed infrared (IR) frequencies based on a harmonic treatment do not match the experimental spectrum. When this nonrigidity is taken into account by using a nonharmonic approximation calculated UMP2/6-31G** IR spectra are in good agreement with the observed spectra of HCCCH and DCCCD.

The potential energy curves for the twisting of tetramethyleneethane in its lowest singlet and triplet states were calculated by the state-specific two-reference Brillouin-Wigner coupled-cluster method with single and double excitations. The calculated potential energy curves are essentially the same as those obtained by the two-determinant CCSD method, and they are also in agreement with the previously reported density functional theory results. Our data bring support for the previously suggested interpretation of experimental data on tetramethyleneethane in the gas phase and in the matrix.