OpenMX uses numerical pseudo-atomic orbitals (PAOs) as basis function to expand one-particle
Kohn-Sham wave functions. The PAO function is given by a product of a radial function and
a real spherical harmonic function as

DATA.PATH ../DFT_DATA19 # default=../DFT_DATA19Both the absolute and relative specifications are possible, and the default is '../DFT_DATA19'.

In an input file for the OpenMX calculation, The basis set is specified by a keyword 'Definition.of.Atomic.Species' as follows:

<Definition.of.Atomic.Species H H5.0-s2p1 H_PBE19 C C5.0-s2p1 C_PBE19 Definition.of.Atomic.Species>where an abbreviation, H5.0-s2p1, of the basis function is introduced. H5.0 stands for the file name of the PAO functions without the file extension which must exist in a directory specified by the keyword 'DATA.PATH', e.g., DFT_DATA19/PAO, and 5.0 implies the cutoff radius of the PAO functions. Also, s2p1 means that two s-state radial functions and one p-state radial function stored in the file are used. In this case, totally five PAO basis functions (2x1+1x3=5) are assigned for 'H'.

Since optimized basis functions are available on the website (http://www.openmx-square.org/) as the database Ver. 2019. We recommend for general users to use these optimized basis functions. But for experts, both the primitive and optimized PAO functions are explained in the subsequent sections.