Scan Keyword
Last Update: 12/31/2000

This calculation type keyword requests that a potential energy surface (PES) scan be done. By default, a rigid PES scan is performed, which consists of single point energy evaluations over a rectangular grid involving selected internal coordinates. The molecular structure must be defined using Z-Matrix coordinates. The number of steps and step size for each variable are specified on the variable definition lines, following the variable's initial value. For example:

R1 1.41  3 0.05

A1 104.5 2 1.0

A2 120.0

This input causes variable R1 to be stepped 3 times by 0.05. Thus four, R1 values (1.41, 1.46, 1.51, and 1.56) will be done for each combination of other variables. Similarly, 3 values for A1 will be used, and A2 will be held fixed at 2.2. All in all, a total of 12 energy evaluations will be performed. Any number of variables can be stepped. The units of the step-sizes are controlled by the Units keyword and default to Angstroms and degrees.

A relaxed PES scan (with geometry optimization at each point) is requested with the Opt keyword.

If any scanning variable breaks symmetry during the calculation, then you must include NoSymm in the route section of the job, or the job will fail with an error.

Restart
Restarts a PES scan calculation. A failed Scan calculation may be restarted from its checkpoint file by simply repeating the route section of the original job, adding the Restart option to the Scan keyword. No other input is required.

Opt

Output files from rigid PES scans conclude with a table summarizing the energy results for the scan:

 Scan completed.

 Summary of the potential surface scan:
   N       R          A          HF     
 ----  ---------  ---------  -----------
    1     0.9600   104.5000    -38.39041
    2     1.0100   104.5000    -38.41306
    3     1.0600   104.5000    -38.42336
    4     0.9600   105.5000    -38.39172
    5     1.0100   105.5000    -38.41430
    6     1.0600   105.5000    -38.42453
    7     0.9600   106.5000    -38.39296
    8     1.0100   106.5000    -38.41547
    9     1.0600   106.5000    -38.42564
   10     0.9600   107.5000    -38.39412
   11     1.0100   107.5000    -38.41657
   12     1.0600   107.5000    -38.42668
 ----  ---------  ---------  -----------

Chapter 8 of Exploring Chemistry with Electronic Structure Methods 304provides a detailed discussion of potential energy surface scans.

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