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Scan

This calculation type keyword requests that a potential energy surface (PES) scan be done. 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 (104.5, 105.5 and 106.5) will be used, and A2 will be held fixed at 120.0. 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 ccan 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 PES scans conclude with a table summarizing the results for the job:

Scan completed.
 
Summary of the potential surface scan:
 N        R          A          SCF     
----  ---------  ---------  -----------
 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 6 of Exploring Chemistry with Electronic Structure Methods [Foresman15, pp.238-44,255-58] provides a detailed discussion of potential energy surface scans.


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