Search results for "Active space"
showing 10 items of 43 documents
Localized molecular orbitals for excited states of polyenals, polyendials, and polyenones
2003
The work is focused on the generation of localized molecular orbitals for excited states. A recently developed a priori method based in a CAS-SCF–type algorithm is applied. The method generates directly localized orbitals and can be applied to multireference wavefunctions. A detailed description of the performance of the method as well as the locality of the MOs for the example of the singlet nπ* (CO) excited state is given. It is in general possible to obtain local orbitals for the doubly occupied and virtual valence orbitals. The partial delocalization of the π* (CO) orbital is discussed, as is the effect of the use of different CAS spaces. The systems under study are polyenals, polyendia…
Multistate active spaces from local CAS-SCF molecular orbitals: the photodissociation of HFCO as an example.
2005
A recently developed algorithm to generate localized molecular orbitals (LMO) is applied to the study of excited states along a photodissociation process. The LMOs allow for the selection of a consistent complete active space (CAS) for the simultaneous description of all the electronic states involved in a multistate process on the basis of simple chemical criteria. The local nature of the orbitals is used to label them in a unique way that does not depend on the molecular geometry. The selection of the electronic configurations of interest for the set of target states on only the basis of the dominant excitations required by the simplest configuration interaction (CI) descriptions for both…
Vertical spectrum of the C2H 2+ system. An open shell (SC)2-CAS-SDCI study
2003
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-specific multireference coupled-cluster theory
2012
The multireference problem is considered one of the great challenges in coupled-cluster (CC) theory. Most recent developments are based on state-specific approaches, which focus on a single state and avoid some of the numerical problems of more general approaches. We review various state-of-the-art methods, including Mukherjee's state-specific multireference coupled-cluster (Mk-MRCC) theory, multireference Brillouin–Wigner coupled-cluster (MR-BWCC) theory, the MRexpT method, and internally contracted multireference coupled-cluster (ic-MRCC) theory. Related methods such as extended single-reference schemes [e.g., the complete active space coupled-cluster (CASCC) theory] and canonical transfo…
Analytic gradients for Mukherjee’s multireference coupled-cluster method using two-configurational self-consistent-field orbitals
2010
Analytic gradients for the state-specific multireference coupled-cluster method suggested by Mahapatra et al. [Mol. Phys. 94, 157 (1998)] (Mk-MRCC) are reported within the singles and doubles approximation using two-configurational self-consistent field (TCSCF) orbitals. The present implementation extends our previous work on Mk-MRCC gradients [E. Prochnow et al., J. Chem. Phys. 131, 064109 (2009)] which is based on restricted Hartree-Fock orbitals and consequently the main focus of the present paper is on the treatment of orbital relaxation at the TCSCF level using coupled-perturbed TCSCF theory. Geometry optimizations on m-arynes and nitrenes are presented to illustrate the influence of t…
Theoretical studies on the spectroscopy of the 7-azaindole monomer and dimer
2001
The absorption and the emission spectra, both fluorescence and phosphorescence, of the 7-azaindole molecule have been studied by means of the complete active space (CAS) SCF method and multiconfigurational second-order perturbation theory (CASPT2). Excitation energies, oscillator strengths, dipole moments, transition dipole moments, and their directions have been computed and the results compared to those of analogous molecules such as indene, indole, and benzimidazole, to get a homogeneous picture of the photophysics of the systems. The absorption and emission of the 7-azaindole dimer and its related tautomer have also been computed in order to get further insight into the double fluoresce…
A Theoretical Study of the Electronic Spectra of N9 and N7 Purine Tautomers
1999
The complete active space (CAS) SCF method and multiconfigurational second-order perturbation theory (CASPT2) have been used to study electronic spectra of the N(9)H and N(7)H tautomers of purine. The calculations include vertical excitation energies, oscillator strengths, dipole moments, and transition moment directions in gas phase. In accord with experiment in nonpolar solvents, the two lowest π → π* excited singlet valence states are predicted to be located at 4.7 and 5.1 eV. The latter is expected to shift to the red in aqueous solutions. A satisfactory interpretation of the electronic spectra above 5.5 eV is obtained if the experimental data are assumed to consist of the superposition…
A theoretical study of the emission spectra of indole and its analogs: indene, benzimidazole, and 7-azaindole
2000
Abstract The complete active space (CAS) SCF method and multiconfigurational second-order perturbation theory (CASPT2) have been used to study the electronic spectra of indole, indene, benzimidazole, and 7-azaindole. The paper is focused on the study of the low-lying valence triplet and singlet electronic states at the optimized geometries of the excited states. The geometries have been optimized by using analytic CASSCF derivatives. CASPT2 point calculations have been performed in order to obtain band origins and relaxed emission energies. The results are analyzed in the context of the complex emission processes, both fluorescence and phosphorescence, displayed by the title compounds, whic…
Nature of Bonding in Group 13 Dimetallenes: a Delicate Balance between Singlet Diradical Character and Closed Shell Interactions
2010
The nature of metal-metal bonding in group 13 dimetallenes REER (E = Al, Ga, In, Tl; R = H, Me, (t)Bu, Ph) was investigated by use of quantum chemical methods that include HF, second order Møller-Plesset perturbation theory (MP2), coupled cluster (CCSD(T)), complete active space with (CASPT2) and without (CAS) second order perturbation theory, and two density functionals, namely, B3LYP and M06-2X. The results show that the metal-metal interaction in group 13 dimetallenes stems almost exclusively from static and dynamic electron correlation effects: both dialuminenes and digallenes have an important singlet diradical component in their wave function, whereas the bonding in the heavier diinde…
Rational Design of Lanthanoid Single-Ion Magnets: Predictive Power of the Theoretical Models
2016
We report two new single‐ion magnets (SIMs) of a family of oxydiacetate lanthanide complexes with D3 symmetry to test the predictive capabilities of complete active space ab initio methods (CASSCF and CASPT2) and the semiempirical radial effective charge (REC) model. Comparison of the theoretical predictions of the energy levels, wave functions and magnetic properties with detailed spectroscopic and magnetic characterisation is used to critically discuss the limitations of these theoretical approaches. The need for spectroscopic information for a reliable description of the properties of lanthanide SIMs is emphasised. SPINMOL ERC-CoG-647301 DECRESIM MAT2014-56143-R CTQ2014-52758-P CTQ2011-2…