Optional keyword

A custom function can have optional arguments. Use the Optional keyword in the list of arguments to declare an optional argument. The optional argument or arguments must be last in the list. 

MolWt.xls is a Function procedure that returns the formula weight from text that can be interpreted as a chemical formula. Only a part of the procedure is reproduced in Chapter 17. This Function procedure illustrates the use of the Optional keyword. See "Instructions for Using MolWt" at the end of this appendix. 

AMPAC can also be run from a shell or queue system using an ASCII input file. The input file format is easy to use. It consists of a molecular structure defined either with Cartesian coordinates or a Z-matrix and keywords for the type of calculation. The program has a very versatile set of options for including molecular geometry and symmetry constraints. 

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. 

Sometimes, an optimization will simply require more steps than the default procedure allots to it. You can increase the maximum number of steps with the MaxCycle option to the Opt keyword (it takes the number of steps as its argument). 

You can specify a different temperature, pressure, and/or set of isotopes for the thermochemical analysis by specifying the Readlsolopes option to the Frecj keyword in the route section. Values for all parameters must then be specified in a separate input section following the molecule specification—and separated from it by a blank line. 

We can compute all of the results except those in the first row by running just three jobs QCISD(T,E4T] calculations on HF and fluorine and a Hartree-Fock calculation on hydrogen (with only one electron, the electron correlation energy is zero). Note that the E4T option to the QCISDfT) keyword requests that the triples computation be included in the component MP4 calculation as well as in the QCISD calculation (they are not needed or computed by default). 

Perform the IRC calculation (requested with the IRC keyword). This job will help you to verify that you have the correct transition state for the reaction when you examine the structures that are downhill from the saddle point. In some cases, however, you will need to increase the number of steps taken in the IRC in order to get closer to the minimum the AAoxPoinb option specifies the number of steps to take in each direction as its argument. You can also continue an IRC calculation by using the lRC=(ReStorhMaxPointe=n) keyword, setting n to some appropriate value (provided, of course, that you have saved the checkpoint file). 

For our initial geometry for the transition structure, we ll detach one hydrogen from the carbon and increase the O-C-H bond angle. We specified the Opt=(TS,CalcFC) keyword in the route section, requesting an optimization to a transition state. The CalcFC option is used to compute the initial force constants, a technique which is generally helpful for transition state optimizations. We ve also included the Freq keyword so that a frequency calculation will automatically be run at the optimized geometry. 

These options to the IRC keyword increase the maximum number of points on each side of the path to 15 and the step size between points to 0.3 amu bohr (30 units of 0.1 amu bohr), where the defaults are 6 steps and 0.1 amu bohr, respectively. The SCF=QC keyword requests the quadratic convergence SCF procedure, a somewhat slower but significantly more reliable SCF procedure. 

The following Gaussian keywords and options are useful for excited state calculations ... 

Since we need to find both triplet and singlet excited states, we ve included the 50-50 option to the CIS keyword. We ve asked for two states of each type, the exact number we require for this well-studied system. When examining new systems, however, it s often a good idea to request slightly more states than you initially want to. allow for degenerate states and other unexpected results. 

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. 

We ve specified five excited states with NSlotes=5 (the reasons will be clear in a moment). The DensitysAll keyword tells Gaussian to perform the population analysis using all available densities the SCF (ground state) density, the Cl one-partide density, and the Cl (Cl-Singles) density. The population analyses using excited state densities will be performed for the first excited state (the default if the Root option is not included), which is the one in which we are interested. 

Change the option for the Density keyword from Current to All m the frequency calculation. 

Gaussian input is designed to be free-format and extremely flexible. For example, it is not case-sensitive, and keywords and options may be shortened to a unique abbreviation. 

The first line of the route section always begins with a pound sign ( ) in the first column. This section specifies the theoretical procedure, basis set, and desired type of calculation. It may also include other keywords. The ordering of keywords is not important. Some keywords require options the following input line illustrates the possible formats for keywords with options ... 

The amount of spacing between items is not significant in Gaussian input. In the route section, commas or slashes may be substituted for spaces if desired (except within parenthesized options, where slashes don t work). For example, the previous route section used a slash to separate the procedure and basis set, spaces to separate other keywords, and commas to separate the options to the Unite keyword. 

QCI methods 117 QCISD keywords 9,114 E4T option 186 quadratic Cl methods 114 quadrupole moment 21 quotation li... 

SCRF keyword 239 input for 239 options 239 SCRF methods... 

Units keyword 14, 34 Bohr option 287 Radian option 287 unrestricted methods 10 user initialization files xxxiii... 

All route sections must include a procedure keyword and a basis set keyword. Additional keywords further specify the type of calculation desired and additional options. 

A menu system was designed for screen presentation. Keywords in the main menu are activated using the cursor keys or the first letter in the keyword. They are then displayed in a highlighted format, and a question connected with their usage is displayed in an accompanying line. The main menu choices are Chemical, Task, Attributes, Search, Match, Options, Print, and Quit. 

Polarization functions may be optionally chosen to be of pure or Cartesian form (by another keyword). In the former case, one includes the expected number of angular-momentum components (i.e., five d orbitals, seven f orbitals, etc.), whereas in the latter case some additional component(s) of lower angular momentum are included (e.g., a Cartesian d set includes five d orbitals plus one s orbital, a Cartesian f set includes seven f orbitals plus three p orbitals, and so forth). 

Health Hotlines directs you to toll-free numbers to over 300 organizations. You can access this database directly at httpy/healthhotlines.nlm.nih.gov/. On this page, you are given the option to search by keyword or by browsing the subject list. When you have received your search results, click on the name of the organization for its description and contact information. 

The submission is now repeated from step 2 with a keyword relevant to the study by typing it in the window for keyword in the advanced option menu. An example of the results produced is shown in Fig. 16.3. 

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