Input Sections

*GEOM Section

Provides the initial molecular geometry in Cartesian coordinates (Ångström).

Format:

*GEOM
Element  X  Y  Z
...
*

Example:

*GEOM
C     -4.108383   -0.269570   -0.105531
C     -3.086048   -0.004314   -0.907012
H     -4.464908    0.456695    0.621271
H     -4.622302   -1.226624   -0.151874
C     -2.326155    1.302381   -0.898892
H     -2.751025   -0.749763   -1.624593
*

You can also reference an external XYZ file:

*GEOM
@geometry.xyz
*

The XYZ file must use standard format (Ångström units, with or without the atom-count / comment header lines).

*TAIL_A and *TAIL_B Sections

These sections inject extra text directly into the QM program's input for state A and state B respectively. Their content is program-specific.

Gaussian — append additional route section keywords:

*TAIL_A
scf(maxcycle=500,xqc)
*

*TAIL_B
scf(maxcycle=500,xqc)
*

ORCA — append additional input blocks (e.g., %tddft, %scf):

*TAIL_A
%tddft
  nroots 5
  iroot  1
end
*

*TAIL_B
%tddft
  nroots 5
  iroot  2
end
*

If no extra keywords are needed, include empty sections:

*TAIL_A
*

*TAIL_B
*

*CONSTR Section

Specifies geometric constraints (fixed internal coordinates) or PES scan specifications. See Constraints & Scans for the full syntax reference.

Example — fix a bond and an angle:

*CONSTR
R 1 2 1.54
A 1 2 3 109.5
*

Example — 1D bond scan:

*CONSTR
s r 1 2 1.0 20 0.05
*

*LST_A and *LST_B Sections

Provide the endpoint geometry for Linear Synchronous Transit (LST) interpolation. When both sections are present, OpenMECP generates an interpolated initial guess by linearly mixing the *GEOM geometry and the LST target.

Format — same as *GEOM (Cartesian coordinates, Ångström):

*LST_A
C  0.0  0.0  0.0
H  1.0  0.0  0.0
*

*LST_B
C  0.0  0.0  0.5
H  1.2  0.0  0.5
*

Atom ordering must match between *GEOM, *LST_A, and *LST_B. OpenMECP applies the Kabsch algorithm to find the optimal rigid-body alignment before interpolating, so pre-alignment of the structures is not required.

See LST Interpolation for full details.