Ab initio theoretical methods for excited states application to ketene and formaldehyde

Technical Report · Sat Aug 01 04:00:00 EDT 1987

OSTI ID:5919396

Allen, W D

Extensive ab initio studies of the X-tilde/sup 1/A/sub 1/, a-tilde/sup 3/A/sub 2/ (/sup 3/A''), A-tilde/sup 1/A/sub 2/ (/sup 1/A''), b-tilde/sup 3/A/sub 1/ (/sup 3/A'), B-tilde/sup 1/B/sub 1/, and 2/sup 1/A/sub 1/ electronic states of ketene have been performed with the goal of providing an essentially complete theoretical description of these low-lying states. Geometrical structures, vertical and adiabatic excitation energies, and vibrational frequencies have been determined for these states, and correlation diagrams and transition states for dissociation to methylene and carbon monoxide have been investigated in detail. The theoretical results are discussed in the context of ketene as a paradigm of five features exhibited in general by certain electronic states: the correlation of excited states to fragments, gross qualitative differences between singlet and triplet states with the same orbital occupancies, conical intersections of potential energy surfaces, variational collapse of trial wave functions, and Rydberg-valence mixing. A new, optimally efficient formalism is presented for the analytic evaluation of two-configuration self-consistent-field configuration interaction (TCSCF-CI) energy first derivatives, and this TCSCF-CI gradient procedure is adapted and applied to singlet excited electronic states of the same symmetry as the ground state. The resulting adaptation, the SCFX-CI gradient procedure, is the simplest means by which dynamical electron correlation can be incorporated effectively for such electronic states, because conventional SCF-CI wave functions are unreliable in these cases due to the possibility of variational collapse. In addition to the 2/sup 1/A/sub 1/ state of ketene, the SCFX-CI and TCSCF-CI gradient methods are applied to the X-tilde/sup 1/A/sub 1/, (..pi.. ..-->.. ..pi..*) 2/sup 1/A/sub 1/, and (n ..-->.. 3p/sub y/) 2/sup 1/A/sub 1/ states of formaldehyde. Specifically, geometrical structures, excitation energies, and harmonic vibrational frequencies obtained with DZP and DZP + R basis sets are reported for H/sub 2/CO.

• Research Organization: Lawrence Berkeley Lab., CA (USA)
• DOE Contract Number: AC03-76SF00098
• OSTI ID: 5919396
• Report Number(s): LBL-23943; ON: DE88001388
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201* -- Chemical & Physicochemical Properties
ALDEHYDES
CONFIGURATION INTERACTION
DISSOCIATION
ELECTRONIC STRUCTURE
ENERGY LEVELS
EXCITED STATES
FORMALDEHYDE
KETENES
METHYLENE RADICALS
ORGANIC COMPOUNDS
ORGANIC OXYGEN COMPOUNDS
RADICALS
SELF-CONSISTENT FIELD