UQCISD method

The E4T option to the UQCISD method keyword tells Gaussian to run fhe component MP4 calcdations at the MP4(SDTQ) level, rather than the default of MP4(SDQ). When you set up the QCISD(T) calculations for methane, include the additional option IOP(2/16=1) in the route section (which says to ignore any symmetry changes during the scan) and also include Gue s=(Always b() for the unrestricted casr. Mix requests that the HOMO and LUMO be mixed so as to destroy alpha-beta and spatial spin symmetries (this option is also useful for producing unrestricted wave functions for singlet systems), and Always says to recompute a new guess wavefunction at each point. 

The initial steps of the reaction between chloro derivatives of ethylene and O3 were studied using UQCISD, UB3LYP, UMP2, and MRMP2 methods with 6-3H-G and aug-cc-PVDZ basis sets. Two possible mechanisms, concerted and stepwise additions of ozone to the double bond, were considered. The energies and geometries of the transition states, rate constants, and kinetic parameters were calculated. ... 

The Pople corrections for higher excitations in CISD calculations result in a method called quadratic (Q) CISD, which can be regarded as an approximation to the coupled-cluster methods discussed in a separate section below. QCISD is available in the Gaussian series of programs, along with analytical derivatives and a method for the estimation of triple excitations called QCISD(T). Note that in Gaussian, CISD on open-shell molecules is performed with UHF MOs. Therefore, for such molecules, the methods just named should really be called UQCISD or UQCISD(T), respectively. 

The variational UCISD method, which results in pure spin states, appears to be able to handle the singlet states of TMM quite well the thermally corrected prediction of AEj-p =14.5 kcal/mol at this level is only 1.6 kcal/mol below the experimental value. However, this agreement is accidental and comes from a fortuitous cancellation of errors, as shown by the greater deviation of the UQCISD and UQCISD(T) values from experiment. 

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