Basis set sections creation window

  Here sections of input files for Gaussian or Gamess/Firefly can be generated by  current molecule. You can take a big text from the basis sets website (https://www.basissetexchange.org), with descriptions of AO functions and ECP data, for all available atoms, then paste this text into Chemcraft, and then Chemcraft will prepare a text with the job specification, which can be inserted into Gaussian or Gamess input file.

 The sections with AO coefficient and ECPs are generated when one chooses one of the pre-defined basis sets (in "Available basis sets" list). The checklist "Current basis set" allows separate basis sets for different atom types to be specified: when one chooses new basis set in "Available basis sets" list, only checked atom types are affected. If "Multiselection" checkbox is checked, one can choose a combination of several basis sets (for example, to use diffuse/polarization basis sets).

  It is not fully clear for us, how good is the idea of combining several basis sets on different atoms in one molecule. At the one hand, different parts of your molecule can require different levels of theory for correct modeling; on the other hand, adding different basis sets to different parts of molecule can produce the basis set superposition error (BSSE), especially for conjugated systems (the electron density shifts to more dense set).

  Sometimes the molecule is symmetrical, but Chemcraft doesn't know this, and it generates input files with C1 symmetry. To avoid this, apply the point group via "Edit/Set point group/" menu items (even if this applying does not change any atomic coordinates in your molecule) before using this window.

  The button "Customize basis sets" allows the collection of pre-defined basis sets to be edited. The button "Add gaussian" allows custom gaussian functions to be added to the current basis set. It opens a dialog window where one can choose the type of gaussian function and values of exponents and contraction coefficients in the format "exponent/coefficient" (if only one digit is specified, contraction coefficient is assigned to 1.0).

  E.g. for Gamess input files, the text consists of sections like "GALLIUM      ! (4s,4p)->[2s,2p]…" (describing basis functions), "TE-ECP GEN 46    4…" (describing ECP). The sections in the text can be given in any sequence. Comment lines are omited by the program. All defined basis sets are stored in "basissets.txt" file in Chemcraft root folder.

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