# Module MMTK.FourierBasis

This module provides a basis that is suitable for the calculation of low-frequency normal modes. The basis is derived from vector fields whose components are stationary waves in a box surrounding the system. For a description see [Hinsen1998].

## Functions

- countBasisVectors()

  Returns the number of basis vectors in a FourierBasis for the given universe and cutoff.

- estimateCutoff()

  Returns an estimate for the cutoff that will yield a basis of nmodes vectors for the given universe. The two return values are the cutoff and the precise number of basis vectors for this cutoff.

## Class FourierBasis: Collective-motion basis for low-frequency normal mode calculations

To be used with MMTK.NormalModes.SubspaceNormalModes.

Constructor: FourierBasis(universe, cutoff)

universe

  the universe for which the basis will be used

cutoff

  the wavelength cutoff. A smaller value means a larger basis.

A FourierBasis object behaves like a sequence of MMTK.ParticleVector objects. The vectors are not orthonormal, because orthonormalization is handled automatically by the class MMTK.NormalModes.SubspaceNormalModes.