Purpose

RELAX drives and controls a non-linear optimization procedure to locate the minimum (or a stationary point) of a function $f(x)$. In TURBOMOLE $f$ is always the electronic energy, and the coordinates $x$ will be referred to as general coordinates. They include

• cartesian atomic coordinates
• internal atomic coordinates
• exponents, contraction coefficients and scaling factors of basis functions
• a global scaling factor (a common scaling factor for all basis set exponents)

The optimization employs an iterative procedure based on gradients $\nabla f$ of the current and, if available, previous iterations. Various procedures can be applied: steepest descent, Pulay's DIIS, quasi-Newton, conjugate gradients, as well as combinations of them. RELAX carries out:

• update of general coordinates
• update of approximate hessians if needed
• conversion of coordinates (internal $\longleftrightarrow$ cartesian)

The mode of operation is chosen by the keywords $optimize and $interconversion and the corresponding options, which will be described in the following sections.