9.4 Potential Energy Scans

It is often useful to scan the potential energy surface (PES), optimizing all other degrees of freedom for each particular value of the scanned variable(s). Such a scan may provide a rough estimate of a pathway between reactant and product—assuming the coordinate(s) for the scan has been chosen wisely—and is often used in development of classical force fields to optimize dihedral angle parameters. Ramachandran plots, for example, are key tools for studying conformational changes of peptides and proteins, and are essentially 2-dimensional torsional scans.

Q-Chem supports one- and two-dimensional PES scans, by setting JOBTYPE equal to PES_SCAN in the $rem section. In addition, a $scan input section with the following format should be specified, in the format below but with no more than two bond-length, bond-angle, or torsional variables specified.

$scan
stre  atom1  atom2  value1 value2 incr
...
bend  atom1  atom2  atom3  value1 value2 incr
...
tors  atom1  atom2  atom3  atom4  value1 value2 incr
...
$end

The first example below demonstrates how to scan the torsional potential of butane, which is a sequence of constrained optimizations with the C1–C2–C3–C4 dihedral angle fixed at $-$ 180 $^\circ$ , $-$ 165 $^\circ$ , $-$ 150 $^\circ$ , $\ldots$ , 165 $^\circ$ , 180 $^\circ$ .

Example 9.215 One-dimensional torsional scan of butane

$molecule
   0 1
   C       1.934574    -0.128781    -0.000151
   C       0.556601     0.526657     0.000200
   C      -0.556627    -0.526735     0.000173
   C      -1.934557     0.128837    -0.000138
   H       2.720125     0.655980    -0.000236
   H       2.061880    -0.759501    -0.905731
   H       2.062283    -0.759765     0.905211
   H       0.464285     1.168064    -0.903444
   H       0.464481     1.167909     0.903924
   H      -0.464539    -1.167976     0.903964
   H      -0.464346    -1.168166    -0.903402
   H      -2.062154     0.759848     0.905185
   H      -2.720189    -0.655832    -0.000229
   H      -2.061778     0.759577    -0.905748
$end

$rem
   JOBTYPE  pes_scan
   METHOD   hf
   BASIS    sto-3g
$end

$scan
   tors 1 2 3 4 -180 180 15
$end

The next example is a two-dimension potential scan. The first dimension is a scan of the C1–C2–C3–C4 dihedral angle from $-$ 180 $^\circ$ to 180 $^\circ$ degree in 30 $^\circ$ intervals; the second dimension is a scan of the C2–C3 bond length from 1.5 to 1.6 in 0.05 increments.

Example 9.216 Two-dimensional torsional scan of butane

$molecule
   0 1
   C       1.934574    -0.128781    -0.000151
   C       0.556601     0.526657     0.000200
   C      -0.556627    -0.526735     0.000173
   C      -1.934557     0.128837    -0.000138
   H       2.720125     0.655980    -0.000236
   H       2.061880    -0.759501    -0.905731
   H       2.062283    -0.759765     0.905211
   H       0.464285     1.168064    -0.903444
   H       0.464481     1.167909     0.903924
   H      -0.464539    -1.167976     0.903964
   H      -0.464346    -1.168166    -0.903402
   H      -2.062154     0.759848     0.905185
   H      -2.720189    -0.655832    -0.000229
   H      -2.061778     0.759577    -0.905748
$end

$rem
   JOBTYPE  pes_scan
   METHOD   hf
   BASIS   sto-3g
$end

$scan
   tors 1 2 3 4 -180 180 30
   stre 2 3 1.5 1.6 0.05
$end