# Psithon Functions: Invoking a Calculation

To allow arbitrarily complex computations to be performed, Psi4 is built upon the Python interpreter, with modifications termed Psithon. Sec. Psithon: Structuring an Input File describes the non-standard Python associated with clean molecule, basis, and option specification in the Psi4 input file. This documentation addresses the pure Python side- what functions allow the efficient compiled code to be run, what functions post-process and interact with that output, and how the ordinary (or ambitious) user can extent Psi4‘s functionality.

- Notes on Options
- Energy
- Property
- Counterpoise Correct
- Optimize
- Frequency
- Database
  - Output
  - Available Databases
- Complete Basis Set
  - Output
  - Extrapolation Schemes
  - Aliases
- Spectroscopic Constants for Diatomics
- Function Intercalls
- Embarrassing Parallelism
- Generation of cube files
  - Introduction
  - Cubeprop Tasks
  - Cubeprop Options