Biomedical subjects

Daniel Maynau

Publications and source records attributed to Daniel Maynau.

7 recordsLinked to original sources

PubMed2007

Selected excitation for CAS-SDCI calculations.

A new selected-configuration interaction method is proposed, based on the use of local orbitals. A corresponding code has been written, which is devoted to CI calculations of rather large systems (about 50-100 carbon-like atoms). Taking advantage of the locality, and then of the fact that interactions vanish when the distance is large, the dimension of the CI space is largely reduced. The determinants that would be created by long range excitations are expected to have a small weight in the wave function and are therefore eliminated. This selected excitation CI space is particularly suited for large molecules. It is tested on large polyene chains and on a transition metal complex. For large enough systems, the CPU time saving is important and, what is more noticeable, calculations that were impossible to perform without selection are feasible in this approach.

Polyenes↗

PubMed2006

Correlated ab initio study of the excited state of the iron-coordinated-mode noninnocent glyoxalbis(mercaptoanil) ligand.

The intriguing and theoretically unresolved magnetic coupling in the Fe(gma)CN (1) compound [gma = glyoxalbis(mercaptoanil)] has been investigated by means of first-principle correlated ab initio calculations. The low-energy spectrum of the complex has been studied using the difference dedicated configuration interaction method, which is a dynamically correlated multiconfigurational method. In agreement with available spectroscopic information, we found that the ground-state doublet is dominated by the coupling between an iron-centered quartet and the first excited triplet on the gma ligand. The open-shell character of the electronic structure of the ligand clarifies its noninnocent nature. The low-energy spectrum reveals the presence of a first excited quartet of different symmetry lying 200 cm(-1) above. The lowest excitation energy in the ground-state symmetry is found at 4790 cm(-1), thus ruling out the simple description of the system based on a Heisenberg Hamiltonian.

Journal Article↗

PubMed2005

Large systems at ab initio multireference level: a cheap treatment thanks to a division into fragments.

Thanks to the use of localized orbitals and the subsequent possibility of neglecting long-range interactions, the linear-scaling methods have allowed to treat large systems at ab initio level. However, the limitation of the number of active orbitals in a complete active space self consistent-field (CASSCF) calculation remains unchanged. The method presented in this paper suggests to divide the system into fragments containing only a small number of active orbitals. Starting from a guess wave function, each orbital is optimized in its corresponding fragment, in the presence of the other fragments. Once all the fragments have been treated, a new set of orbitals is obtained. The process is iterated until convergence. At the end of the calculation, a set of active orbitals is obtained, which is close to the exact CASSCF solution, and an accurate CASSCF energy can be estimated.

Journal Article↗

PubMed2005

A combined freeze-and-cut strategy for the description of large molecular systems using a localized orbitals approach.

A technique to reduce the computational effort in calculating ab initio energies using a localized orbitals approach is presented. By exploiting freeze strategy at the self-consistent field (SCF) level and a cut of the unneeded atomic orbitals, it is possible to perform a localized complete active space (CAS-SCF) calculation on a reduced system. This will open the possibility to perform ab initio treatments on very large molecular systems, provided that the chemically important phenomena happen in a localized zone of the molecule. Two test cases are discussed, to illustrate the performance of the method: the cis-trans interconversion curves for the (7Z)-13 ammoniotridec-7-enoate, which demonstrates the ability of the method to reproduce the interactions between charged groups; and the cisoid-transoid energy barrier for the aldehydic group in the C13 polyenal molecule.

Journal Article↗

PubMed2004

Can density functional methods be used for open-shell actinide molecules? Comparison with multiconfigurational spin-orbit studies.

The geometries, electronic structures, and vibrational frequencies of two isoelectronic compounds PuO(2)(2+) and PuN(2) have been studied in detail at the density functional theory (DFT) and multiconfigurational ab initio levels of theory. Dynamic correlation was taken into account using second-order perturbation theory (CASPT2) and the variational difference-dedicated configuration interaction method for comparison with the results of the DFT study. Spin-orbit effects were included within the framework of an effective uncontracted spin-orbit configuration-interaction method which considers electron correlation effects and spin-orbit coupling on equal footing. The twelve lowest f-f electronic transitions are reported. The electronic ground state of both systems is found to be the Omega=4 component of (3)H(g). We thus disagree with an earlier assignment of the ground state of PuN(2) [E. F. Archibong and A. K. Ray, J. Mol. Struct: THEOCHEM 530, 165 (2000)]. Spin-orbit effects are small on both the geometry and vibrational frequencies of the ground states of PuO(2)(2+) and PuN(2), but they completely change the distribution of electronically excited states. A comparison of results obtained with the two classes of methods allows us to demonstrate that an unambiguous assignment of the electronic ground state and electronic spectra requires the use of multireference methods including spin-orbit coupling. Single-reference methods such as DFT provide a reasonable description of the electronic properties of ground states of these open-shell systems, and therefore also of their structural and vibrational properties. The experimental antisymmetric stretching frequency of matrix-isolated PuN(2) is reproduced well by both CASPT2 and DFT calculations; generalized gradient approximation formulations of DFT are more successful than hybrid versions in this respect. Ground-state properties of UO(2) (2+), UN(2), UO(2), PuO(2) (2+), and PuN(2) are compared and discussed.

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PubMed2003

Vertical spectrum of the C2H2+ system. An open shell (SC)2-CAS-SDCI study.

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.

Journal Article↗

PubMed2002

Reducing CAS-SDCI space. Using selected spaces in configuration interaction calculations in an efficient way.

A new method is presented, which allows an important reduction of the size of some Configuration Interaction (CI) matrices. Starting from a Complete Active Space (CAS), the numerous configurations that have a small weight in the CAS wave function are eliminated. When excited configurations (e.g., singly and doubly excited) are added to the reference space, the resulting MR-SDCI space is reduced in the same proportion as compared with the full CAS-SDCI. A set of active orbitals is chosen, but some selection of the most relevant excitations is performed because not all the possible excitations act as SDCI generators. Thanks to a new addressing technique, the computational time is drastically reduced, because the new addressing of the selected active space is as efficient as the addressing of the CAS. The presentation of the method is followed by two test calculations on the N(2) and HCCH molecules. For the N(2) the FCI results are taken as a benchmark reference. The outer valence ionization potentials of HCCH are compared to the experimental values. Both examples allow to test the accuracy of the MR-SDCI compared to that of the corresponding CAS-SDCI, despite the noticeable reduction of the CI space. The algorithm is suitable for the dressing techniques that allow for the correction of the size-extensivity error. The corrected results are also shown and discussed.

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