Syntax

PSI3 input files are case-insensitive and free-format, with a grammar designed for maximum flexibility and relative simplicity. Input values are assigned using the structure:

keyword = value

where keyword is the parameter chosen (e.g., convergence) and value has one of the following data types:

• string: A character sequence surrounded by double-quotes. Example: basis = "cc-pVDZ"
• integer: Any positive or negative number (or zero) with no decimal point. Example: maxiter = 100
• real: Any floating-point number. Example: omega = 0.077357
• boolean: true, false, yes, no, 1, 0.
• array: a parenthetical list of values of the above data types. Example: docc = (3 0 1 1).

Note that the input parsing system is general enough to allow multidimensional arrays, with elements of more than one data type. A good example is the z-matrix keyword:

zmat = (
  (O)
  (H 1 r)
  (H 1 r 2 a)
)

For z-matrices, z-matrix variables, and Cartesian coordinates, it is also possible to discard the inner parentheses. The following is equivalent in this case:

zmat = (
  O
  H 1 r
  H 1 r 2 a
)

Keywords must grouped together in blocks, based on the module or modules that require them. The default block is labelled psi:, and most users will require only a psi: block when using PSI3. For example, the following is a simple input file for a single-point CCSD energy calculation on H$_2$O:

psi: (
  label = "6-31G**/CCSD H2O"
  wfn = ccsd
  reference = rhf
  jobtype = sp
  basis = "6-31G**"
  zmat = (
     O
     H 1 r 
     H 1 r 2 a 
  )
  zvars = (
     r 1.0 
     a 104.5 
  )
)

In this example, the psi: identifier collects all the keywords (of varying types) together. Every PSI3 module will have access to every keyword in the psi: block by default. One may use other identifiers (e.g., ccenergy:) to separate certain keywords to be used only by selected modules. For example, consider the keyword convergence, which is used by several PSI3 modules to determine the convergence criteria for constructing various types of wave functions. If one wanted to use a high convergence cutoff for the PSI3 SCF module but a lower cutoff for the coupled cluster module, one could modify the above input:

psi: (
  ...
  convergence = 7
)
scf:convergence = 12

Note that, since we have only one keyword associated with the scf: block, we do not need to enclose it parentheses.

Some additional aspects of the PSI3 grammar to keep in mind:

• The ``%'' character denotes a comment line, i.e. any information following the ``%'' up to the next linebreak is ignored by the program.
• Anything in between double quotes (i.e. strings) is case-sensitive.
• Multiple spaces are treated as a single space.