If no action code is included, the default action is to add the specified coordinate. The atom numbers are followed by a one-character code letter indicating the coordinate modification to be performed the action code is sometimes followed by additional required parameters as indicated above. Atom numbering begins at 1, and any dummy atoms are not counted.
N1, N2, N3 and N4 are atom numbers or wildcards (discussed below).
Lines in a ModRedundant input section use the following syntax: AddRedundant is synonymous with ModRedundant. When used in conjunction with QST2 or QST3, a ModRedundant input section must follow each geometry specification. This option requires a separate input section following the geometry specification. OPTIONS TO MODIFY THE INITIAL GEOMETRYĪdd, delete or modify redundant internal coordinate definitions (including scan and constraint information) before performing the calculation. Note that CASSCF=SlaterDet is needed in order to locate a conical intersection between a singlet state and a triplet state. Search for a conical intersection or avoided crossing using the state-averaged CASSCF method. Requests optimization to a saddle point of order N using the Berny algorithm. Requests optimization to a transition state rather than a local minimum, using the Berny algorithm. Note that the atoms must be specified in the same order within the three structures. This option requires the reactant, product, and initial TS structures as input, specified in three consecutive groups of title and molecule specification sections. Search for a transition structure using the STQN method. Note that the atoms must be specified in the same order in the two structures. This option requires the reactant and product structures as input, specified in two consecutive groups of title and molecule specification sections. SELECTING THE OPTIMIZATION GOALīy default, optimizations search for a local minimum. For a review article on optimization and related subjects, see. See the examples for sample input for and output from this method.īasic information as well as techniques and pitfalls related to geometry optimizations are discussed in detail in chapter 3 of Exploring Chemistry with Electronic Structure Methods. The order of the atoms must be identical within all molecule specifications. QST2 requires two molecule specifications, for the reactants and products, as its input, while QST3 requires three molecule specifications: the reactants, the products, and an initial structure for the transition state, in that order. This method is requested with the QST2 and QST3 options. This method will converge efficiently when provided with an empirical estimate of the Hessian and suitable starting structures. Like the default algorithm for minimizations, it performs optimizations by default in redundant internal coordinates. Schlegel and coworkers, uses a quadratic synchronous transit approach to get closer to the quadratic region of the transition state and then uses a quasi-Newton or eigenvector-following algorithm to complete the optimization. Gaussian includes the STQN method for locating transition structures. The default algorithm for all methods lacking analytic gradients is the eigenvalue-following algorithm ( Opt=EF). An brief overview of the Berny algorithm is provided in the final subsection of this discussion. For the Hartree-Fock, CIS, MP2, MP3, MP4(SDQ), CID, CISD, CCD, CCSD, QCISD, BD, CASSCF, and all DFT and semi-empirical methods, the default algorithm for both minimizations (optimizations to a local minimum) and optimizations to transition states and higher-order saddle points is the Berny algorithm using GEDIIS in redundant internal coordinates (corresponding to the Redundant option). Analytic gradients will be used if available. The geometry will be adjusted until a stationary point on the potential surface is found. This keyword requests that a geometry optimization be performed.
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