Vertical spectrum of the C₂H system. An open shell (SC)²‐CAS‐SDCI study

Abstract

The open shell (SC)(2)-CAS-SDCI method along with a basis set of atomic natural orbitals (ANO) has been applied for calculating the main ionization potentials of acetylene, as well as the manifold of excited states of the different symmetries up to 32 eV. In this method, the single and double excitations of a CAS space are generated and the corresponding CI matrix is corrected by means of the (SC)(2) procedure that cancels the size-extensivity error and adds some high order contributions. The mean absolute error for the outer-valence X (2)Pi(u)(1pi(u) (-1)), A (2)Sigma(g) (+)(3sigma(g) (-1)), and B (2)Sigma(u) (+)(2sigma(u) (-1)) states, and the inner-valence C (2)Sigma(g) (+)(2sigma(g) (-1)) state is 0.1 eV. The excited states of C(2)H(2) (+) corresponding to the Sigma(g) (+), Sigma(u) (+), Pi(g), Pi(u), Delta(g), and Delta(u) symmetries are reported and their composition is discussed. The results are thoroughly compared to the best available multireference CI calculations. Recent multichannel CI results by Wells and Lucchese [J Chem Phys 1999, 110, 6365] have been used also as a guide for the discussion of the results. Discrepancies in the description of many multiconfigurational states by means of the (SC)(2)-CAS-SDCI wave function as compared to previous large MR-CI calculations are significant and are, consequently, remarked on. A large mixing of the 3sigma(g) (-1) and 2sigma(g) (-1) processes is found in the A (2)Sigma(g) (+) and C (2)Sigma(g) (+) states, provided that the basis set is augmented with Rydberg functions.

Ultrafast hydrogen migration in acetylene cation driven by non-adiabatic effects

“…Such calculations exist already in the literature. [43][44][45][46][47][48][49][50][51][52] Our focus is on the dynamics of acetylene that follows after an outer valence ionization. Given the large computational effort associated with the dynamics calculations, it is necessary to find a balance between accuracy and the simulation cost.…”

Description of a poorly differentiated carcinoma within the brainstem of a white whale (Delphinapterus leucas) from magnetic resonance images and histological analysis

“…The high accuracy of the open‐shell (SC) 2 ‐CAS‐SDCI method for the outer‐valence and higher inner‐valence transitions of small molecules10, 43 is confirmed for the main peaks of acetylene. In particular, the inner‐valence C 2 Σ vertical IP is accurately calculated.…”

“…Following Wells and Lucchese,35 we show in Table 1 the two different values from the EMS29 and XPS28 studies. Apart from these energy discrepancies, which are commonly found in other small systems (e.g., N and NH43), the nature of these states is clear.…”

“…The extension of the method to systems where the wave function of this dressing reference state is dominated by a unique open‐shell determinant has been formulated in our group. It has been applied to the calculation of vertical outer valence and main inner‐valence IPs of a number of small molecules 10, 43. Hence, we give here only a brief summary of the procedure.…”

Search of truncation of (N−1) electron basis containing full connected triple excitations in computing main and satellite ionization potentials via Fock‐space coupled cluster approach

“…The results of the inner valence level (Table 3) have also been compared with experiment[85] and other current generation theoretical results obtained via nondiagonal 2ph‐TDA[94] and (SC) 2 ‐CAS‐SDCI[95] methods. Although EOM‐CCSDT in principle is more accurate than EIP‐VUMRCCSDτ, the related computation is too expensive and quite slow.…”