Input files

The maps code needs two input files: one that specifies the geometry of the parent lattice (lat.in) and one that provides the parameters of the first-principles calculations (xxxx.wrap, where xxxx is the name of the first-principles code used). The clear separation between the thermodynamic and first-principles calculations is a distinguishing feature of atat that enables the package to be easily interfaced with any first-principles code. Table 5.1 gives two annotated examples of a lattice geometry input file. The package includes ready-made lattice files for the common lattice types (e.g. bcc, fcc, hcp). It also includes an utility that automatically constructs multiple lattice geometry input files for common lattices. For instance,

 makelat Al,Ti fcc,bcc,hcp

creates 3 subdirectories containing the appropriate input files for each specified lattice.

Example 1: hcp Ti-Al system

Table 5.1: Examples of lattice geometry input file lat.in. Typically, the coordinate system entry is used to define the conventional unit cell so that all other entries can be specified in the normalized coordinates that are the most natural for the symmetry of the lattice. The input lattice parameters do not need to be exact, as the first-principles code will optimize them.

3.1 3.1 5.062 90 90 120	(Coordinate system: $a$ $b$ $c$ $\alpha$ $\beta$ $\gamma$ notation)
1 0 0	(Primitive unit cell: one vector per line
0 1 0	expressed in multiples of the above coordinate
0 0 1	system vectors)
0 0 0 Al,Ti	(Atoms in the lattice)
0.6666666 0.3333333 0.5 Al,Ti

Example 2: rocksalt CaO-MgO pseudobinary system

4.1 4.1 4.1 90 90 90

0 0.5 0.5
0.5 0 0.5
0.5 0.5 0
0 0 0 Ca,Mg	(“Active” atoms in the lattice)
0.5 0.5 0.5 O	(“spectator” ion)

The first-principles input file is usually less than 10 lines long, thanks to the dramatic improvements in the user-friendliness of most modern first-principles codes. For instance, in the case of the widely used VASP code [13,12], a typical input file is given in Table 5.2. Examples of such input files are provided with the package. Note that atat contains a utility that enables the automatic construction of $k$-point meshes from a single parameter defining the desired target $k$-point density, the number of $k$-point per reciprocal atom (KPPRA).

Table 5.2: Examples of first-principles code input file (example given for the vasp code). It is especially important to verify that the KPPRA parameter is set sufficiently large for the system under study.

[INCAR]
PREC = high
ENMAX = 200
ISMEAR = -1
SIGMA = 0.1
NSW=41
IBRION = 2
ISIF = 3	(See vasp manual for a description of the above 6 parameters.)
KPPRA = 1000	(Sets the $k$-point density (K Point Per Reciprocal Atom))
DOSTATIC	(Performs a “static run” -- see vasp manual)