Molcas Forum

Anugraha

Member

Registered: 2025-12-17

Posts: 22

Constraint geometry optimization:Syntax error

Dear Molcas Developers,

I am perfoming a CASSCF constraint geometry optimization and i need to fix the difference in distance between two bonds during geometry optimization.
In the Gateway module of my input i have given the following constraint block.

 Constraints
 r1 = bond O7 H8
 r2 = bond H8 O9
 a = 1.0 r1 - 1.0 r2
Values
 a = -0.9 angstrom
end of constraints

However the calculation terminates with an error like this :

++ Constraints section
                                  CONSTRAINTS

********************************************************************************
 R1 = BOND O7 H8
 R2 = BOND H8 O9
 A = 1.0 R1 - 1.0 R2
VALUES
 A = -0.9 ANGSTROM
********************************************************************************


 *************************************************************
 * Values of the primitive constraints                       *
 *************************************************************
  R1     : Bond Length=  0.977221 / angstrom  1.846680 / bohr
  R2     : Bond Length=  1.865635 / angstrom  3.525539 / bohr
 ###############################################################################
 ###############################################################################
 ###                                                                         ###
 ###                                                                         ###
 ###    ERROR: Error in DefInt2                                              ###
 ###                                                                         ###
 ###                                                                         ###
 ###############################################################################
 ###############################################################################
  Line contains syntax error!
 A = 1.0 R1 - 1.0 R2                                                                                                   
--- Stop Module: slapaf at Wed Feb 11 06:54:54 2026 /rc=_RC_INPUT_ERROR_ ---

I would be thankful if anyone can suggest how to resolve this issue.

Thank You
Anugraha

Ignacio

Administrator

From: Uppsala

Registered: 2015-11-03

Posts: 1,204

Re: Constraint geometry optimization:Syntax error

Have you tried this?

a = 1.0 r1 + -1.0 r2

Anugraha

Member

Registered: 2025-12-17

Posts: 22

Re: Constraint geometry optimization:Syntax error

Have you tried this?

a = 1.0 r1 + -1.0 r2

Yes Sir . It is also giving the same error

********************************************************************************
 R1 = BOND O7 H8
R2 = BOND H8 O9
A = 1.0 R1 + -1.0 R2
VALUES
A = -0.9 ANGSTROM
********************************************************************************


 *************************************************************
 * Values of the primitive constraints                       *
 *************************************************************
  R1     : Bond Length=  0.977221 / angstrom  1.846680 / bohr
 R2       : Bond Length=  1.865635 / angstrom  3.525539 / bohr
 ###############################################################################
 ###############################################################################
 ###                                                                         ###
 ###                                                                         ###
 ###    ERROR: Error in DefInt2                                              ###
 ###                                                                         ###
 ###                                                                         ###
 ###############################################################################
 ###############################################################################
  Line contains syntax error!
A = 1.0 R1 + -1.0 R2                                                                                                    
--- Stop Module: slapaf at Wed Feb 11 08:45:15 2026 /rc=_RC_INPUT_ERROR_ ---

Ignacio

Administrator

From: Uppsala

Registered: 2015-11-03

Posts: 1,204

Re: Constraint geometry optimization:Syntax error

If I understand the docs correctly, for constraints you should specify the function in the VALUE part, i.e.:

Constraints
 r1 = bond O7 H8
 r2 = bond H8 O9
Values
 1.0 r1 - 1.0 r2 = -0.9 angstrom
End of Constraints

Anugraha

Member

Registered: 2025-12-17

Posts: 22

Re: Constraint geometry optimization:Syntax error

If I understand the docs correctly, for constraints you should specify the function in the VALUE part, i.e.:

Constraints
 r1 = bond O7 H8
 r2 = bond H8 O9
Values
 1.0 r1 - 1.0 r2 = -0.9 angstrom
End of Constraints

Thank you so much Sir. It is working.