Dear Adam,

You can easily fix any coordinate along optimization.For example, consider the methane and assume one of C-H bond distances needs to be fixed. It is worth to note that geometry should be saved as (gjf) format. The initial (input) file can be as follows:

0 1

C

H 1 B1

H 1 B2 2 A1

H 1 B3 3 A2 2 D1 0

H 1 B4 3 A3 2 D2 0

B1 1.07000000

B2 1.07000000

B3 1.07000000

B4 1.07000000

A1 109.47120255

A2 109.47125080

A3 109.47121829

D1 -119.99998525

D2 120.00000060

Now, assume B1 needs to be fixed. You should remove B1 among variables and paste it at the end of variable after a blank line:

0 1

C

H 1 B1

H 1 B2 2 A1

H 1 B3 3 A2 2 D1 0

H 1 B4 3 A3 2 D2 0

B2 1.07000000

B3 1.07000000

B4 1.07000000

A1 109.47120255

A2 109.47125080

A3 109.47121829

D1 -119.99998525

D2 120.00000060

B1 1.07000000

Now you can save and run the calculation. It must be noted that inclusion of z-matrix in the route section is mandatory:

#hf/3-21g opt=z-matrix

In the output file :

GradGradGradGradGradGradGradGradGradGradGradGradGradGradGradGradGradGrad

Berny optimization.

Initialization pass.

----------------------------

! Initial Parameters !

! (Angstroms and Degrees) !

---------------------- ----------------------

! Name Value Derivative information (Atomic Units) !

------------------------------------------------------------------------

! B2 1.07 estimate D2E/DX2 !

! B3 1.07 estimate D2E/DX2 !

! B4 1.07 estimate D2E/DX2 !

! A1 109.4712 estimate D2E/DX2 !

! A2 109.4712 estimate D2E/DX2 !

! A3 109.4712 estimate D2E/DX2 !

! D1 -120.0 estimate D2E/DX2 !

! D2 120.0 estimate D2E/DX2 !

! B1 1.07 Frozen !

------------------------------------------------------------------------

as can bee seen, B1 is frozen during the optimization. I hope you find it helpful.

Sincerely,

Saeed

Why do you not use relative coordinate? I think there is no meaning for your purpose because the whole moiety except H moves freely in this calculation. It means the H atom moves freely compared with other atoms.

Dear Adam,

You can easily fix any coordinate along optimization.For example, consider the methane and assume one of C-H bond distances needs to be fixed. It is worth to note that geometry should be saved as (gjf) format. The initial (input) file can be as follows:

0 1

C

H 1 B1

H 1 B2 2 A1

H 1 B3 3 A2 2 D1 0

H 1 B4 3 A3 2 D2 0

B1 1.07000000

B2 1.07000000

B3 1.07000000

B4 1.07000000

A1 109.47120255

A2 109.47125080

A3 109.47121829

D1 -119.99998525

D2 120.00000060

Now, assume B1 needs to be fixed. You should remove B1 among variables and paste it at the end of variable after a blank line:

0 1

C

H 1 B1

H 1 B2 2 A1

H 1 B3 3 A2 2 D1 0

H 1 B4 3 A3 2 D2 0

B2 1.07000000

B3 1.07000000

B4 1.07000000

A1 109.47120255

A2 109.47125080

A3 109.47121829

D1 -119.99998525

D2 120.00000060

B1 1.07000000

Now you can save and run the calculation. It must be noted that inclusion of z-matrix in the route section is mandatory:

#hf/3-21g opt=z-matrix

In the output file :

GradGradGradGradGradGradGradGradGradGradGradGradGradGradGradGradGradGrad

Berny optimization.

Initialization pass.

----------------------------

! Initial Parameters !

! (Angstroms and Degrees) !

---------------------- ----------------------

! Name Value Derivative information (Atomic Units) !

------------------------------------------------------------------------

! B2 1.07 estimate D2E/DX2 !

! B3 1.07 estimate D2E/DX2 !

! B4 1.07 estimate D2E/DX2 !

! A1 109.4712 estimate D2E/DX2 !

! A2 109.4712 estimate D2E/DX2 !

! A3 109.4712 estimate D2E/DX2 !

! D1 -120.0 estimate D2E/DX2 !

! D2 120.0 estimate D2E/DX2 !

! B1 1.07 Frozen !

------------------------------------------------------------------------

as can bee seen, B1 is frozen during the optimization. I hope you find it helpful.

Sincerely,

Saeed

It is impossible to freeze cartesian coordinates as told by Mauro.