How to only optimize the distance between the centers of the rings in pi-pi stacking by NWCHEM?

zmatrix

#

# Centers of rings at distance SD

#

 X1

 X2 1 SD

#

# Ring one

#

 C 1 CX 2 ANG1

 C 1 CX 2 ANG1 3 BD1

 C 1 CX 2 ANG1 4 BD1

 C 1 CX 2 ANG1 5 BD1

 C 1 CX 2 ANG1 6 BD1

 C 1 CX 2 ANG1 7 BD1

 H 1 HX 2 ANG1 3 TCH1

 H 1 HX 2 ANG1 4 TCH1

 H 1 HX 2 ANG1 5 TCH1

 H 1 HX 2 ANG1 6 TCH1

 H 1 HX 2 ANG1 7 TCH1

 H 1 HX 2 ANG1 8 TCH1

1. 
   
2. Ring two
   
3. 
   

 C 2 CX 1 ANG2  3 TR1

 C 2 CX 1 ANG2 15 BD2

 C 2 CX 1 ANG2 16 BD2

 C 2 CX 1 ANG2 17 BD2

 C 2 CX 1 ANG2 18 BD2

 C 2 CX 1 ANG2 19 BD2

 H 2 HX 1 ANG2 15 TCH2

 H 2 HX 1 ANG2 16 TCH2

 H 2 HX 1 ANG2 17 TCH2

 H 2 HX 1 ANG2 18 TCH2

 H 2 HX 1 ANG2 19 TCH2

 H 2 HX 1 ANG2 20 TCH2

 variables

  SD 2.0

 constants

  CX 1.3

  HX 2.3

  ANG1 90.0

  ANG2 90.0

  BD1  60.0

  BD2  60.0

  TCH1  0.0

  TCH2  0.0

  TR1   0.0

end

* library sto-3g

maxiter 100