1,056 research outputs found
Canonical transformation theory from extended normal ordering
The canonical transformation theory of Yanai and Chan [J. Chem. Phys. 124, 194106 (2006)] provides a rigorously size-extensive description of dynamical correlation in multireference problems. Here we describe a new formulation of the theory based on the extended normal ordering procedure of Mukherjee and Kutzelnigg [J. Chem. Phys. 107, 432 (1997)]. On studies of the water, nitrogen, and iron oxide potential energy curves, the linearized canonical transformation singles and doubles theory is competitive in accuracy with some of the best multireference methods, such as the multireference averaged coupled pair functional, while computational timings (in the case of the iron oxide molecule) are two to three orders of magnitude faster and comparable to those of the complete active space second-order perturbation theory. The results presented here are greatly improved both in accuracy and in cost over our earlier study as the result of a new numerical algorithm for solving the amplitude equations
Canonical transformation theory from extended normal ordering
The canonical transformation theory of Yanai and Chan [J. Chem. Phys. 124, 194106 (2006)] provides a rigorously size-extensive description of dynamical correlation in multireference problems. Here we describe a new formulation of the theory based on the extended normal ordering procedure of Mukherjee and Kutzelnigg [J. Chem. Phys. 107, 432 (1997)]. On studies of the water, nitrogen, and iron oxide potential energy curves, the linearized canonical transformation singles and doubles theory is competitive in accuracy with some of the best multireference methods, such as the multireference averaged coupled pair functional, while computational timings (in the case of the iron oxide molecule) are two to three orders of magnitude faster and comparable to those of the complete active space second-order perturbation theory. The results presented here are greatly improved both in accuracy and in cost over our earlier study as the result of a new numerical algorithm for solving the amplitude equations
The ground-state spectroscopic constants of Be_2 revisited
Extensive ab initio calibration calculations combined with extrapolations
towards the infinite-basis limit lead to a ground-state dissociation energy of
Be_2, D_e=944 \pm 25 1/cm, substantially higher than the accepted experimental
value, and confirming recent theoretical findings. Our best computed
spectroscopic observables (expt. values in parameters) are G(1)-G(0)=223.7
(223.8), G(2)-G(1)=173.8 (169 \pm 3), G(3)-G(2)=125.4 (122 \pm 3), and
B_0=0.6086 (0.609) 1/cm; revised spectroscopic constants are proposed.
Multireference calculations based on a full valence CAS(4/8) reference suffer
from an unbalanced description of angular correlation; for the utmost accuracy,
a CAS(4/16) reference including the orbitals is required, while for
less accurate work a CAS(4/4) reference is recommended. The quality of computed
coupled cluster results depends crucially on the description of connected
triple excitations; the CC5SD(T) method yields unusually good results because
of an error compensation.Comment: Chem. Phys. Lett., in pres
Multireference approaches for excited states of molecules
Understanding the properties of electronically excited states is a challenging task that becomes increasingly important for numerous applications in chemistry, molecular physics, molecular biology, and materials science. A substantial impact is exerted by the fascinating progress in time-resolved spectroscopy, which leads to a strongly growing demand for theoretical methods to describe the characteristic features of excited states accurately. Whereas for electronic ground state problems of stable molecules the quantum chemical methodology is now so well developed that informed nonexperts can use it efficiently, the situation is entirely different concerning the investigation of excited states. This review is devoted to a specific class of approaches, usually denoted as multireference (MR) methods, the generality of which is needed for solving many spectroscopic or photodynamical problems. However, the understanding and proper application of these MR methods is often found to be difficult due to their complexity and their computational cost. The purpose of this review is to provide an overview of the most important facts about the different theoretical approaches available and to present by means of a collection of characteristic examples useful information, which can guide the reader in performing their own applications
Accurate quantum chemical calculations
An important goal of quantum chemical calculations is to provide an understanding of chemical bonding and molecular electronic structure. A second goal, the prediction of energy differences to chemical accuracy, has been much harder to attain. First, the computational resources required to achieve such accuracy are very large, and second, it is not straightforward to demonstrate that an apparently accurate result, in terms of agreement with experiment, does not result from a cancellation of errors. Recent advances in electronic structure methodology, coupled with the power of vector supercomputers, have made it possible to solve a number of electronic structure problems exactly using the full configuration interaction (FCI) method within a subspace of the complete Hilbert space. These exact results can be used to benchmark approximate techniques that are applicable to a wider range of chemical and physical problems. The methodology of many-electron quantum chemistry is reviewed. Methods are considered in detail for performing FCI calculations. The application of FCI methods to several three-electron problems in molecular physics are discussed. A number of benchmark applications of FCI wave functions are described. Atomic basis sets and the development of improved methods for handling very large basis sets are discussed: these are then applied to a number of chemical and spectroscopic problems; to transition metals; and to problems involving potential energy surfaces. Although the experiences described give considerable grounds for optimism about the general ability to perform accurate calculations, there are several problems that have proved less tractable, at least with current computer resources, and these and possible solutions are discussed
On the calculation of complete dissociation curves of closed-shell pseudo-onedimensional systems through the multireference method of increments
The Method of Increments (MoI) has been employed using a multireference
approach to calculate the dissociation curve of beryllium ring-shaped clusters
Be of different sizes. Benchmarks obtained through different single and
multireference methods including the ab initio Density Matrix Renormalization
Group (DMRG) were used to verify the validity of the MoI truncation which
showed a reliable behavior for the whole dissociation curve. Moreover we
investigated the size dependence of the correlation energy at different
distances in order to extrapolate the values for the periodic chain and to
discuss the transition from a metal-like to a insulating-like behavior of the
wave function through quantum chemical considerations
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