LST Interpolation

Linear Synchronous Transit (LST) interpolation generates an initial MECP guess by linearly mixing between two endpoint geometries. This is useful when the MECP lies somewhere between a known reactant geometry and a known product geometry.

Setup

Add *LST_A and *LST_B sections alongside the standard *GEOM:

*GEOM
C  -4.10   -0.27   -0.11
H  -4.46    0.46    0.62
*

*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
*

When both *LST_A and *LST_B sections are present, OpenMECP:

  1. Applies the Kabsch algorithm to optimally align both structures to the *GEOM reference (removes translational and rotational differences).
  2. Generates a sequence of 10 interpolation points along the LST path.
  3. Prints the energy profile along the interpolated path.
  4. Asks for interactive confirmation before proceeding with MECP optimization.

Interpolation Methods

MethodDescription
LST (default)Linear Synchronous Transit — linear mixing of coordinates
QSTQuadratic Synchronous Transit — uses midpoint approximation

Note: QST currently uses a midpoint approximation for the third geometry. Full QST with an explicit user-supplied midpoint is planned for a future release.

Kabsch Alignment

The Kabsch algorithm finds the optimal rotation matrix that minimizes the RMSD between corresponding atoms of the two structures. This ensures that the interpolation path follows the shortest physically meaningful route between the two geometries, avoiding spurious rotations.

No manual pre-alignment of the structures is required — OpenMECP handles this automatically.

Output

After LST interpolation:

OutputDescription
Interpolation energy profilePrinted to stdout: coordinate vs. E₁, E₂
running_dir/lst_*.inpInput files for each interpolation point
{basename}_mecp.xyzFinal MECP geometry after optimization

Tips

  • LST is most effective when the two endpoint geometries differ mainly in bond lengths or angles, not overall topology.
  • If the energy profile shows a clear minimum-energy crossing along the LST path, the starting geometry for MECP optimization will be close to the MECP, leading to faster convergence.
  • Use print_level = 2 to see the full interpolated path during the LST stage.