Guess keyword

Gaussian includes a facility for automatically generating a starting structure for a transition state optimization based upon the reactants and products that the transition structure connects, known as the STQN method. This feature is requested with the QST2 option to the Opt keyword. Input files using this option will include two title and molecule specification sections. The facility generates a guess for the transition structure which is midway between the reactants and products, in terms of redundant internal coordinates. 

When this initial guess is poor, you need a more sophisticated—albeit more expensive—means of generating the force constants. This is especially important for transition state optimizations. Gaussian provides a variety of alternate ways of generating them. Here are some of the most useful associated keywords consult the Gaussian User s Reference for a full description of their use ... 

The first job step computes the energies of the three lowest excited states. The second job step uses its results to begin the optimization by including the Read option to the CIS keyword, Geom=Check, and Guess=Read (and of course the commands to name and save the checkpoint file). The Freq keyword computes the frequencies at the optimized structure. 

The GuesssAlter keyword is used to ensure that the orbitals of interest are included in the active space. This keyword allows orbitals to be swapped in order within the initial guess. A prior run with Gue s=Only and/or Pop=Full can be used to quickly determine the orbital symmetries and decide which ones should be placed within the active space. 

To run a BOVB calculation smoothly, it is advisable to start from an appropriate guess function, which may be, for example, a preliminary VBSCF wave function. In the XMVB program, the BOVB procedure sets up automatically by coding the keyword bovb in the input. As with the VBSCF method, the spectator orbitals in the BOVB method may be defined as either localized or delocalized, resulting in the L- and D-BOVB methods, respectively. 

The keyword vbscf means that a calculation of VBSCF type will be performed. As the VB orbitals are all localized, this will be an L-VBSCF calculation. A guess function is automatically provided at the end of the input. However, no guess function is required for this calculation, which converges very fast after a few iterations. 

Replacing the keyword vbscf by bovb would do automatically an L-BOYB calculation. In such a case, doing a preliminary L-VBSCF calculation and using its orbitals as a guess is recommended, albeit not compulsory. 

Specific electronic states may also be specified using the Guess=Alter keyword, which allows you to explicitly designate orbital occupancies. See the Gaussian User s Reference for details. 

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 nbo aonbo=cs keyword requests storing the CIS-level NBOs in the shared checkpoint file where guess=read will read them as initial guess for the CAS/NBO job. Note that CAS identifies the 1st excited state as nroot=2 whereas CIS uses root=l for this state. Note also that the Gaussian open-shell CAS implementation faik to provide relevant spin density information to NBO, forcing spin-averaged NBO description of reduced accuracy. This restriction strongly detracts from the potential usefulness of CAS calculations for excited-state analysis. However, illustrative use of this method allows one to see how one can still obtain useful NBO-based descriptors of the excited state despite loss of spin information. 

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