using the NRLMOL

**February 6, 2008**

The CLUSTER file is the main input file of NRLMOL. It contains the minimal information to set up the calculation. See below the CLUSTER file for the Nitrogen atom:

GGA-PBE*GGA-PBE # Exchange-correlation parametrization

GRP # Point group of system

1 # No. of atoms

0.0 0.0 0.0 7 ALL # x,y,z coordinates, Atomic number, ALL means all electron

0.0 3.000 # Charge and Moment

Spherical atomic calculation. Input auto generated by Perl script,

Rajendra Zope, EL Paso, TX 79912. Wed Feb 6 15:17:59 CST 2008

We will now describe the input structure of this file.

- The first line is
**GGA-PBE*GGA-PBE**. It means that the exchange-correlation interactions in the systems are modeled within the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) parametrization. This is the default functional used in NRLMOL. A few other functional are also available. - The second line is
**GRP**. It refers to point group symmetry. The**GRP**means the symmetry point group operations will be read from the**GRPMAT**file. For spherical atoms the**GRPMAT**will contain a unit matrix. You need not generate**GRPMAT**file. It will be created automatically. - The third line contains
**1**. It specifies the number of atoms in calculations. We're running atomic calculations. So number of atoms is 1. For molecules this number will change. See tutorial on molecular calculations for more details. - The line following third line contains the cartesian position in atomic units of the atom, its atomic number, and string
**ALL**. In this example, we are dealing with only one atom, which can be conveniently put at the origin (0, 0, 0). Followed by the xyz coordinates is the atomic charge. The example listed is for Nitrogen whose atomic number is 7. The string**ALL**means include all (that is**7**in this case) electrons into calculations. - The fifth line in example has two fields. The first field is 0.0 which means perform calculation for the neutral atom. If it is 1 then the calculations will be performed for singly charged cation of N atoms. The next field which is 3 in this example corresponds to number of unpaired electron in the system. The nitrogen has three unpaired electrons in the 2p orbitals.
- Lines after
**Charge and Moment**line are ignored.

Now, copy the example input in the file called **CLUSTER** and run the calculation for nitrogen atom using the following sequences of commands at the prompt.

$ rm -f GEOCNVRG SYMBOL

$ ./cluster > print.SCF

Open the **GEOCNVRG** file and write down the energy. If you have done everything correctly then it should be -54.535482 Hartree.

Now, browse through the file **print.SCF** and look at the energies printed at each iteration. The difference between total energy in two successive iterations should diminish as the self consistent process progresses. When this difference is smaller than the threshold value set in ** SCFDAT** the SCF process will stop.

Also, look at the **EVALUES** file in which Kohn-Sham eigenvalues and occupation numbers are printed. Try to understand the SCF process, occupation numbers, degeneracy of orbitals.