# Notes on Options

Note

The Python options referred to in the Psithon Functions: Invoking a Calculation section below are placed as arguments to a Python function (like `energy()`), not in `set` blocks or commands.

Note

All PSI4 keyword names and values are insensitive to case, both those that are placed in `set` blocks and as Python function arguments. The one exception is documented for the subset option in the `database()` function, where case structure must match the database file.

Note

Boolean options can be specified by `yes`, `on`, `true`, or `1` for affirmative and `no`, `off`, `false`, or `0` for negative, all insensitive to case.

Note

The derivative level type for `optimize()` and `frequency()` functions can be specified by `energy`, `none`, or `0` for 0th derivative, `gradient`, `first`, or `1` for 1st derivative, and `hessian`, `second`, or `2` for 2nd derivative. For finite difference, as opposed to analytic, derivatives, the POINTS option can be increased to `5` for greater accuracy at increased cost.

Note

Function option for the Psithon function called by the current function; the default is usually `energy()`. See Sec. Function Intercalls for a fuller description. Note that the value of the keyword is a Python object and so is not wrapped in quotes like a string.

Note

The molecule to be acted upon by the current function; the default is the “active” molecule, which is the nearest preceeding molecule declared in a `molecule mymol {...}` block or in an `activate(mymol)` statement. Note that the value of this keyword (`mymol` in the example) is a Python object and so is not wrapped in quotes like a string. Technically, this is a `Molecule` object.