Self-consistent Field Multireference Research Articles

Low-lying excited-states of 5-benzyluracil

A numerical study is reported concerning the first and second singlet excited-states of 5-benzyluracil using the multireference self-consistent field (state-averaged CASSCF) method. The vertical excitation energies of low-lying excited-states were characterized using the SA-CASSCF method, as well as using higher-level methods, such as CASPT2, MRCI and EOM-CCSD. The local minima and conical intersections found on the potential energy surfaces (PESs) were characterized in terms of molecular geometry and natural population analysis. Different relaxation pathways on the PESs are identified and discussed by comparing with the similar pathways found for the individual monomers of uracil and benzene. The molecule can be thought of as a model system for the study of crosslink reaction between DNA and proteins induced by UV light.

Variational geminal-augmented multireference self-consistent field theory: Two-electron systems

We introduce a geminal-augmented multiconfigurational self-consistent field method for describing electron correlation effects. The approach is based on variational optimization of a MCSCF-type wave function augmented by a single geminal. This wave function is able to account for some dynamic correlation without explicit excitations to virtual molecular orbitals. Test calculations on two-electron systems demonstrate the ability of the proposed method to describe ionic and covalent electronic states in a balanced way, i.e., including the effects of both static and dynamic correlation simultaneously. Extension of the theory to larger systems will potentially provide an alternative to standard multireference methods.

Effects of strong electron correlations in Ti 8C 12 Met-Car

The results of multireference configuration interaction (MRCI) with single and double excitations and single reference coupled cluster (CCSD(T)) calculations on Ti 8C 12 metallocarbohedryne (Met-Car) are reported. The distortions of the T d structure to D 2d and C 3v structures due to the Jahn–Teller effect are studied. It is shown that the Ti 8C 12 wave function has significant multireference character. The choice of the active space for multireference self-consistent field (MCSCF) calculations is discussed. The failure of multireference perturbation theory with a small active space is attributed to multiple intruder states. A new, novel type of MCSCF calculation, ORMAS (occupation restricted multiple active spaces) with a large active space are carried out for several electronic states of Ti 8C 12. The Jahn–Teller distorted D 2d 1A 1 (nearly T d) structure is predicted to be the Ti 8C 12 ground state. Predictions of the Ti 8C 12 ionization potential with different ab initio methods are presented.

Intensity and wavelength control of a single molecule reaction: Simulation of photodissociation of cold-trapped MgH+

Photodissociation of cold magnesium hydride ions MgH(+) leading to either Mg(+)+H or Mg+H(+) is simulated from first principles. The purpose is to study the possibility of single molecule control of the products in the presence of two laser fields. The system evolves on four electronic potential-energy curves, X(1) Sigma, A(1) Sigma, B(1) Pi, and C(1) Sigma. These potential-energy curves are calculated from first principles using multireference self-consistent field theory. The accuracy of the electronic potential curves has been checked by calculating the energies of the rovibrational eigenstates and comparing them to experimental findings. The photodissociation dynamics has furthermore been simulated by solving the time-dependent Schrodinger equation. It is shown that the branching ratio of the two dissociation channels, Mg(+)+H or Mg+H(+), can be controlled by changing the intensity and wavelength of the two driving laser fields.

Ab initio dynamic dipole polarizabilities for O2, its photoabsorption spectrum in the Schumann–Runge region, and long-range interaction coefficients for its dimer

Perturbed states representing the response to an external electric field are obtained in the multireference averaged coupled pair functional formalism based on orbitals which are optimized in a perturbed multireference self-consistent field procedure. For each perturbing operator perturbed wave functions for several frequencies of the perturbing field are obtained simultaneously. From these sets of perturbed states reduced spectra are derived which represent effective oscillator strength distributions. The broad shape of the Schumann–Runge continuum leads to a poor description of the dynamic polarizability for frequencies approaching the first pole when only vertical transitions are considered. To account for this effect the absorption spectrum in the Schumann–Runge region is calculated and the polarizability is described up to the range of anomalous dispersion. The dynamic polarizabilities and the interaction coefficients are found to be in excellent agreement with experimental data where it is available. Significant differencies of up to 20% are observed between our anisotropic interaction coefficients and earlier estimates obtained from semiempirical anisotropic dipole oscillator strength distributions.

Ab initio dynamic multipole polarizabilities and hyperpolarizabilities of H2O and the long-range interaction coefficients for its dimer

Perturbed states representing the response to an external electric field are obtained from multi-reference configuration interaction wavefunctions. The multi-configurational reference function is designed to represent the ground state as well as the perturbed and even the first excited states on equal footing, i.e., orbital optimization is performed in a perturbed multi-reference self-consistent field procedure. For various perturbing operators, several perturbed wavefunctions for different frequencies of the perturbing field are obtained simultaneously with the corresponding lowest excited state. From these sets of perturbed and excited states, reduced spectra are derived which represent effective oscillator strength distributions and serve to calculate dynamic polarizabilities, hyperpolarizabilities, and long-range interaction coefficients. The results for both the dynamic polarizabilities and the interaction coefficients are found to be in excellent agreement with experimental data and other accurate theoretical values.

The ground state of the Fe2 molecule

The Fe2 molecule is a typical transition metal dimer which has a rather large dissociation energy and a small bond distance compared with the inter-nuclear distance in the crystalline metal. We have investigated the Fe2 molecule with multireference self-consistent-field (MCSCF) and multireference configuration interaction (CI) calculations. The dissociation energy (De), the equilibrium nuclear distance (Re), and the zero-point frequency (ωe) were calculated (with observed in parentheses) as 1.57 (1.30±0.22) eV, 2.06 (1.87 to 2.02) Å, and 260.9 (299.6) cm−1, respectively. Thus the agreement between experiment and calculation is very satisfactory, and is a marked improvement on previous theoretical studies. The contribution of the d electrons to the bonding is important and a proper description of correlation effects among the d electrons is indispensable.