Input File Structure

Now let's have a closer look at this input file. Here are some basic rules on how the input is read.

The input is organized in sections which start with &NAME and end with &END. Everything outside those sections is ignored. Also all keywords have to be in upper case or else they will be ignored. The sequence of the sections does not matter, nor does the order of keywords, except where noted in the manual. A minimal input file must have a &CPMD, &SYSTEM, and an &ATOMS section. For more details on the input syntax, please have a look at section 11 of this manual.

&INFO
 Isolated Hydrogen Molecule.
 Single Point Calculation.
&END

This input file starts with an (optional) &INFO section. This section allows you to put comments about the calculation into the input file and they will be repeated in the output file. This can be very useful to identify and match your input and output files.

&CPMD
  OPTIMIZE WAVEFUNCTION
  CONVERGENCE ORBITALS
   1.0d-7
&END

This first part of &CPMD section instructs the program to do a single point calculation (see OPTIMIZE WAVEFUNCTION) with a tight wavefunction convergence criterion (see CONVERGENCE, the default is 1.0d-5).

&SYSTEM
 SYMMETRY
   SIMPLE CUBIC
 CELL
  16.00 1.0 1.0  0.0  0.0  0.0
 CUTOFF
  70.0
&END

The &SYSTEM section contains various parameters related to the simulations cell and the representation of the electronic structure. The keywords SYMMETRY, CELL and CUTOFF are required and define the (periodic) symmetry, shape, and size of the simulation cell, as well as the plane wave energy cutoff (i.e. the size of the basis set), respectively. We define a primitive cubic cell with a lattice constant of 16 a.u. ( $\approx 8.47$Å ). The cell has to be large enough to avoid significant interactions of the hydrogen molecule and its electron structure with its periodic neighbors. In CPMD all calculations are periodic.

&DFT
  FUNCTIONAL LDA
&END

The &DFT section is used to select the density functional (FUNCTIONAL) and related parameters. In this case we go with the local density approximation (which also is the default).

&ATOMS
*H_MT_LDA.psp
  LMAX=S
   2
 8.800    8.000  8.000
 7.200    8.000  8.000
&END

Finally the &ATOMS section is needed to specify the atom coordinates and the pseudopotential(s), that are used to represent them. We provide the position for two hydrogen atoms (in a.u.). The detailed syntax of the pseudopotential specification is a bit more complicated and will not be discussed here. If you want to know more, please have a look at section 11.5 of this manual.