# Overview of auto_diff module¶

The auto_diff module provides Fortran derived types that support automatic differentiation via operator overloading. Users will not generally need to interact with this module, but it can be used within run_star_extras to make derivatives easier to calculate (e.g. in the implicit hooks like other_surface).

Usage is by writing use auto_diff at the top of a module or routine. This imports types such as auto_diff_real_4var_order1, which supports first-order derivatives with respect to up to four independent variables. A variable of this type could be declared via:

type(auto_diff_real_4var_order1) :: x


This variable then holds five fields: x%val stores the value of x. x%d1val1 stores the derivative of x with respect to the first independent variable. x%d1val2 is the same for the second independent variable, and so on. All d1val_ fields are initialized to zero when the variable is first set. Once an auto_diff variable it initialized, all mathematical operations can be performed as they would be on a real(dp) variable. auto_diff variables also interoperate with real(dp) and integer types. So for instance in the following f%d1val1 stores df/dx and f%d1val2 stores df/dy.:

x = 3d0
x%d1val1 = 1d0
y = 2d0
y%d1val2 = 1d0
f = exp(x) * y + x + 4


Similar types are included supporting higher-order and mixed-partial derivatives. These derivatives are accessed via e.g. d2val1 ($$\partial^2 f/\partial x^2$$), d1val1_d2val2 ($$\partial^3 f/\partial x \partial y^2$$).

An additional special type auto_diff_real_star_order1 provides support for first-order derivatives accessed using arrays. This type contains a value (x%val) and an array of first partial derivatives with respect to 27 independent variables (x%d1Array(1:27)). This type is meant to make it easy to write equations and then, after the fact, change the basis of independent variables or re-index them. The number 27 is chosen to provide as many independent variables as the MESA/star solver uses, as this type is meant for use in writing the equations of stellar evolution.