A Computational Study of Adducts Between Atomic Chlorine and Carbon Dioxide, Carbonyl Sulfide and Carbon Disulfide Page: 3
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2. COMPUTATIONAL APPROACH
Geometries of CXY (X and Y = 0 and/or S) and CICXY are optimized at the
self-consistent field (Hartree-Fock) level  with the 3-21G and 3-21G(*)
split-valence basis sets of atomic orbitals (the latter set includes d-type
polarization functions on S and Cl)  using analytical gradients .
Spin-restricted HF is used for closed-shell species , and unrestricted HF
for open-shell species . Harmonic frequencies are calculated at these
geometries to verify that true energy minima are located. These frequencies
are scaled by a factor of 0.9 to approximate v0 before calculation of
zero-point vibrational energies (ZPE) . Total electronic energies at each
HF/3-21G(*) geometry are then calculated by means of fourth order
MlIler-Plesset perturbation theory with contributions from single, double,
triple and quadruple substitutions to account for electron correlation, with
frozen core electrons , applied using. the 3-21G(*) basis set. Combination
of MP theory with this basis set has been employed previously e.g. by Gordon
and co-workers . For open-shell species there is a possibility of
spin-contamination which affects the calculated energies: this is accounted
for by spin-projection  and the results are denoted as
PMP4/3-21G(*)//HF/3-21G(*) energies. All calculations are carried out using
the Gaussian 88 program  on a VAX 6310 computer.
3. RESULTS AND DISCUSSION
Structures and frequencies of CXY molecules
A necessary condition for the calculational method is that it reproduce
the known properties of the parent CXY molecules. The linear geometries
calculated for 12 02, COS and CS2 are summarized in Table I. It may be seen
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Marshall, Paul. A Computational Study of Adducts Between Atomic Chlorine and Carbon Dioxide, Carbonyl Sulfide and Carbon Disulfide, article, November 15, 1991; [Amsterdam, Netherlands]. (digital.library.unt.edu/ark:/67531/metadc725816/m1/3/: accessed January 23, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT College of Arts and Sciences.