In order to allow for an anisotropic distribution function the starting point will be a MAXWELL distribution

(5.6) |

where

(5.7) |

is the standard deviation.

To get the value of the coefficient the normalization

(5.8) |

is used. By using spherical polar coordinates and the substitution the integral can be written as

(5.9) |

Setting gives

(5.10) |

where the GAMMA function has been used

(5.11) |

The coefficient and the distribution function is found to be

(5.12) | ||

(5.13) |

An anisotropic MAXWELL distribution function is obtained by splitting the argument of the exponential function into three separate components

(5.14) | ||

(5.15) |

Since the odd moments of this distribution function are zero, current flow would not be possible. To allow for current flow, the distribution function is shifted

(5.16) |

Again, the diffusion approximation is applied which assumes that the displacement is small, ,

(5.17) | ||

The symmetric and the anti-symmetric part are found to be

(5.18) | ||

(5.19) |

The equations for the current density and the energy flux density are obtained by calculating the statistical averages of the tensors occurring in eqns. (2.76) and (2.77)

The elements outside the trace are zero due to the symmetry properties of , introduced by the diffusion approximation. The statistical averages present in eqns. (5.20) to (5.23) all have one of the forms , , or , and will be integrated in the following.

(5.23) |

(5.24) |

(5.25) |

Inserting these results into eqns. (5.20) to (5.23) finally yields

(5.26) |

which reflects eqn. (5.5),

(5.27) |

and

(5.28) | ||

The flux equations of the anisotropic energy transport model thus become

(5.29) |

(5.30) |

and their one-dimensional projection onto an arbitrary direction reads

(5.31) | ||

(5.32) |

In terms of the general flux equation (3.41)

(5.33) |

the quantities , , and read

(5.34) | ||||||||

(5.35) |

Note that the discretization eqn. (3.61) can be used without modification.