Hello Rupali,

As suggested by Meghna, try using internal coordinates, which involve setting the IFZMAT and FVALUE flags. Below is an example of constraining the carbon-carbon bond to 1.617 A in ethane. Hope this helps.

Cheers,

Karl

-------

$CONTRL ICHARG=0 SCFTYP=RHF RUNTYP=OPTIMIZE

COORD=CART UNITS=ANGS NZVAR=18 $END

$SYSTEM TIMLIM=1440 $END

$BASIS GBASIS=N31 NGAUSS=6 NDFUNC=1 $END

$GUESS GUESS=HUCKEL $END

$STATPT NSTEP=200 $END

$ZMAT AUTO=.TRUE. DLC=.TRUE. IFZMAT(1)=1,1,5 FVALUE(1)=1.617 $END

$DATA

Input from previous run

C1

C 6.0 0.7635864473 0.0000455705 0.0000588013

H 1.0 1.1560764595 -0.8533215607 0.5443361854

H 1.0 1.1564018899 -0.0447113948 -1.0109874412

H 1.0 1.1564359254 0.8979332380 0.4669513446

C 6.0 -0.7637891607 0.0003518050 -0.0001014680

H 1.0 -1.1562793874 0.8537177724 -0.5443804951

H 1.0 -1.1566046033 0.0451106598 1.0109447170

H 1.0 -1.1566384272 -0.8975369199 -0.4669921308

$END

Dear Rupali,

You should think of adding the order of the coordinate you want to freeze as ifreez=1,2 to freeze the 1st and 2nd coordinates of the first atom. 

To add to Elie's answer, 

ifreez(1)=1,2 will freeze the 1st and 2nd coordinate (i.e. x and y coordinate) of the first atom if the coordinate system is Cartesian. However, if you are using internal coordinates then the same command will freeze the 1st and 2nd degrees of freedom of the molecule. Using internal coordinates is more useful if you want to constraint a few structural parameters such as bond lengths, or angles or dihedral angles.

Hello Rupali,

As suggested by Meghna, try using internal coordinates, which involve setting the IFZMAT and FVALUE flags. Below is an example of constraining the carbon-carbon bond to 1.617 A in ethane. Hope this helps.

Cheers,

Karl

-------

$CONTRL ICHARG=0 SCFTYP=RHF RUNTYP=OPTIMIZE

COORD=CART UNITS=ANGS NZVAR=18 $END

$SYSTEM TIMLIM=1440 $END

$BASIS GBASIS=N31 NGAUSS=6 NDFUNC=1 $END

$GUESS GUESS=HUCKEL $END

$STATPT NSTEP=200 $END

$ZMAT AUTO=.TRUE. DLC=.TRUE. IFZMAT(1)=1,1,5 FVALUE(1)=1.617 $END

$DATA

Input from previous run

C1

C 6.0 0.7635864473 0.0000455705 0.0000588013

H 1.0 1.1560764595 -0.8533215607 0.5443361854

H 1.0 1.1564018899 -0.0447113948 -1.0109874412

H 1.0 1.1564359254 0.8979332380 0.4669513446

C 6.0 -0.7637891607 0.0003518050 -0.0001014680

H 1.0 -1.1562793874 0.8537177724 -0.5443804951

H 1.0 -1.1566046033 0.0451106598 1.0109447170

H 1.0 -1.1566384272 -0.8975369199 -0.4669921308

$END

Karl N Kirschner Sir,

Thank you for detailed information.

In addition to above, you can use FMO method also. Which allow to use IFREEZ (1)=1, 2,...