# 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
- Single-Point Energy — `energy()`
- Property
- Basis Set Superposition Corrections
- Geometry Optimization — `optimize()` and `gradient()`
- Harmonic Vibrational Analysis and Visualization of Normal Modes — `frequency()` and `hessian()`
  - Visualization of Normal Modes
- Database — `database()`
  - Output
  - Available Databases
- Complete Basis Set
  - Output
  - Extrapolation Schemes
  - Aliases
- Spectroscopic Constants for Diatomics
- Function Intercalls
- Embarrassing Parallelism
- Generation of Cube Files — `cubeprop()`
  - Introduction
  - Cubeprop Tasks
  - Keywords
- Orbital Visualization with VMD
  - Script Prerequisites
  - Running the Script
  - Script Options