# 4.2.15. EXPBAS¶

This module is used to manipulate orbital files. By default it is used to expand the orbital file to a larger basis set. This feature is available only for generally contracted basis sets (ANO or CC). The original orbital file INPORB is needed. By default, the Runfile, RUNFIL1, from the small calculation is also needed. One runs also the GATEWAY module with the larger basis set to obtain the corresponding runfile, RUNFIL2. The program will produce the file EXPORB, which can be used as the input orbital file for calculations using the larger basis set. The DESYmmetrize keyword is used to transform orbitals with symmetry to C1 symmetry. If one wants to desymmetrize the orbitals without expanding the basis set, the keyword NOEXpand must be used. When the DESYmmetrize keyword is used, in combination with NOEXpand, only the INPORB file is needed. One runs the GATEWAY with the same basis set of the original orbital file to obtain the corresponding runfile. The program will produce the DESORB file. In any subsequent calculation using the DESORB file, modified Cartesian coordinates are required, the ones internally generated by Molcas and printed out in the GATEWAY section of the output.

## 4.2.15.1. Dependencies¶

The EXPBAS requires one InpOrb file and two Runfiles produced by other programs. If the keyword NOEXpand is used, only the INPORB file is needed.

## 4.2.15.2. Files¶

### 4.2.15.2.1. Input files¶

INPORB, an orbital file for the original basis set.

Two more files are needed only if the basis set expansion is to be performed: RUNFIL1, the runfile for the small basis set. RUNFIL2, the runfile for the larger basis set.

### 4.2.15.2.2. Output files¶

EXPBAS generates the files: EXPORB which contains orbitals, orbital energies, occupation numbers, and type indices for the larger basis set. DESORB which contains orbitals, orbital energies, occupation numbers, and type indices for the desymmetrized basis set.

## 4.2.15.3. Input¶

&EXPBAS


### 4.2.15.3.1. Input example¶

>>COPY name.InpOrb INPORB
>>COPY name.RunFil1 RUNFIL1
>>COPY name.Runfil2 RUNFIL2
&EXPBAS


### 4.2.15.3.2. Optional general keywords¶

NOEXpand

Deactivates the basis set expansion.

DESYmmetrize

Activates the basis set desymmetrization.

FILEorb

The next line specifies the filename containing the original orbitals that will be expanded to the larger basis set. By default a file named INPORB will be used.

Comments. For the basis set expansion procedure, this is the strategy:

1. Do a calculation (SCF, DFT, CASSCF, etc.) with a small basis set. Save the produced orbital file and the runfile as: name.Inporb and name.Runfil1.

2. Run a GATEWAY to create the second runfile, which is then saved as name.Runfil2

3. Run EXPBAS as indicated above. The file name.ExpOrb is produced. This file can be used as input for large basis set calculations. Note that the inactive and active orbitals will have the same place in the list as with the smaller basis set, so they can be easily identified.