PNO–CI (pair natural orbital configuration interaction) and CEPA–PNO (coupled electron pair approximation with pair natural orbitals) calculations of molecular systems. I. Outline of the method for closed‐shell states

1975 ◽  
Vol 62 (4) ◽  
pp. 1225-1234 ◽  
Author(s):  
R. Ahlrichs ◽  
H. Lischka ◽  
V. Staemmler ◽  
W. Kutzelnigg
Keyword(s):  
Configuration Interaction ◽  
Electron Pair ◽  
Closed Shell ◽  
Pair Approximation ◽  
Natural Orbital ◽  
Natural Orbitals ◽  
Molecular Systems ◽  
Orbital Configuration

Assessment of the Direct Generalized Bloch Approach B0: Application to the Li and Be Atoms and the Molecules LiH, BeH, and the Phenolate Anion

2005 ◽  
Vol 70 (8) ◽  
pp. 1272-1314
Author(s):  
Holger Meissner
Keyword(s):  
Simple Model ◽  
Configuration Interaction ◽  
Closed Shell ◽  
Experimental Results ◽  
Basis Sets ◽  
Coupled Cluster ◽  
Photoactive Yellow Protein ◽  
Energy Calculations ◽  
Molecular Systems ◽  
Reasonable Approach

Besides the necessity of the development of sophisticated methods to calculate correlation energies - be it the coupled-cluster (CC) or the configuration-interaction (CI) methods and their various approaches - one also accentuate the need for efficient and less demanding methods in the area of medium and large molecular systems. Therefore, this article proposes a computational efficient and in our opinion reasonable approach for the calculation of correlation energies for medium and even larger molecules. This approach, named B0, based on the so-called direct generalized Bloch (DGB) equation which has already been successfully applied to small systems. Within those considerations the B0 approach showed promising results so that further investigations are worthwhile. Here, as a further step in the assessment of this method we apply the B0 approach to the Li and Be atoms as well as the LiH and BeH molecules. Molecules which show open and closed shell characteristics in the equilibrium and in the case of dissociation as well. The results are compared with CC and CI and experimental results if available. Since this results are encouraging even when considering small basis sets and with the prospect of larger molecular systems, therefore, we perform also B0 energy calculations for the low-lying states of the phenolate anion which for instance can be used in a simple model of the photoactive yellow protein (PYP) chromophore.

scholarly journals A test of composite natural orbitals for benzene

1992 ◽  
Vol 70 (2) ◽  
pp. 532-536 ◽  
Author(s):  
Yuichi Yamamoto ◽  
Takeshi Noro ◽  
Kimio Ohno
Keyword(s):  
Excited State ◽  
Configuration Interaction ◽  
Benzene Molecule ◽  
Natural Orbital ◽  
Excitation Energies ◽  
Natural Orbitals ◽  
Experimental Values ◽  
Ci Calculations ◽  
Good Agreement

Approximate natural orbitals (NO's) of a larger system can be constructed from the NO's of smaller fragment systems. These orbitals, called composite NO's (CNO's) are expected to be useful in configuration interaction (CI) calculations. The effectiveness of these NO's is shown for the benzene molecule. This molecule is considered a combination of three ethylenes. The CI calculations were carried out for the S1 – S3 and T1 – T3 states. We take into account single and double excitations from σ and π electrons in the CI calculations. The calculated excitation energies are in good agreement with the experimental values Keywords: benzene, π–π* excited state, composite natural orbital, ionic and covalent, SDCI.

Sign in / Sign up

Export Citation Format

Share Document