4.2. Surface Scans

Relaxed surface scans are an invaluable tool when exploring potential energy surfaces. They allow the user to vary one or more coordinates (bond lengths, angles, or dihedrals) while relaxing the remaining degrees of freedom. This section outlines the theoretical background, gives practical examples, and summarizes relevant input keywords.

4.2.1. Theory

In a relaxed surface scan, a selected coordinate is varied over a defined range, and at each point, a constrained geometry optimization is performed. This can reveal transition states, reaction mechanisms, or approximate energy barriers. ORCA supports scans over bond lengths (B), bond angles (A), and dihedral angles (D).

Multiple coordinates can also be scanned either independently (nested loops) or simultaneously using the Simul_Scan keyword, the latter reducing computational cost.

4.2.2. Simple Example

The following input scans the bond length between C and O from 1.35 Å to 1.10 Å in 12 equidistant steps. Each step involves a constrained geometry optimization.

! B3LYP SV(P) Opt
%geom 
     scan
        B 0 1 = 1.35, 1.10, 12 # C-O distance that will be scanned
     end
end

* int 0 1
    C  0 0 0 0.0000   0.000    0.00
    O  1 0 0 1.3500   0.000    0.00
    H  1 2 0 1.1075 122.016    0.00
    H  1 2 3 1.1075 122.016  180.00
*

Tip

  • As in constrained geometry optimizations it is possible to start the relaxed surface scan with a different scan parameter than the value present in your molecule. But keep in mind that this value should not be too far away from your initial structure.

4.2.3. Complex Example

A more challenging example is shown below. Here, the H-atom abstraction step from \(\text{CH}_4\) to OH-radical is computed with a relaxed surface scan (vide supra). The job was run as follows:

! B3LYP SV(P) Opt SlowConv NoTRAH
%geom 
    scan 
        B 1 0 = 2.0, 1.0, 15 
    end 
end
* int 0 2
 C     0   0   0   0.000000     0.000     0.000
 H     1   0   0   1.999962     0.000     0.000
 H     1   2   0   1.095870   100.445     0.000
 H     1   2   3   1.095971    90.180   119.467
 H     1   2   3   1.095530    95.161   238.880
 O     2   1   3   0.984205   164.404    27.073
 H     6   2   1   0.972562   103.807    10.843
*

It is obvious that the reaction is exothermic and passes through an early transition state in which the hydrogen jumps from the carbon to the oxygen. The structure at the maximum of the curve is probably a very good guess for the true transition state that might be located by a transition state finder.

You will probably find that such relaxed surface scans are incredibly useful but also time consuming. Even the simple job shown below required several hundred single point and gradient evaluations (convergence problems appear for the SCF close to the transition state and for the geometry once the reaction partners actually dissociate – this is to be expected). Yet, when you search for a transition state or you want to get insight into the shapes of the potential energy surfaces involved in a reaction it might be a good idea to use this feature. One possibility to ease the burden somewhat is to perform the relaxed surface scan with a “fast” method and a smaller basis set and then do single point calculations on all optimized geometries with a larger basis set and/or higher level of theory. At least you can hope that this should give a reasonable approximation to the desired surface at the higher level of theory – this is the case if the geometries at the lower level are reasonable.

../../_images/613.svg

Fig. 4.1 Relaxed surface scan for the H-atom abstraction from CH\(_4\) by OH-radical (B3LYP/SV(P)).

4.2.4. Multidimensional Scans

After several requests from our users ORCA now allows up to three coordinates to be scanned within one calculation.

! B3LYP SV(P) Opt
%geom 
     scan
        B 0 1 = 1.35, 1.10, 12 # C-O distance that will be scanned
        B 0 2 = 1.20, 1.00, 5  # C-H distance that will be scanned
        A 2 0 1 = 140, 100, 5  # H-C-O angle that will be scanned
     end
end

* int 0 1
    C  0 0 0 0.0000   0.000    0.00
    O  1 0 0 1.3500   0.000    0.00
    H  1 2 0 1.1075 122.016    0.00
    H  1 2 3 1.1075 122.016  180.00
*

Note

  • For finding transition state structures of more complicated reaction paths ORCA now offers its very efficient NEB-TS implementation (see section Nudged Elastic Band Method).

  • 2-dimensional or even 3-dimensional relaxed surface scans can become very expensive - e.g. requesting 10 steps per scan, ORCA has to carry out 1000 constrained optimizations for a 3-D scan.

  • The results can depend on the direction of the individual scans and the ordering of the scans.

Simultaneous multidimensional scans, in which all scan coordinates are changed at the same time, can be requested via the following keyword (which brings the cost of a multidimensional relaxed surface scan down to the cost of a single relaxed surface scan):

%geom 
  Scan B 0 1 = 3, 1, 15 end
  Scan B 1 2 = 1, 3, 15 end
  Simul_Scan true
end

4.2.5. Multiple XYZ File Scans

A different type of scan is implemented in ORCA in conjunction with relaxed surface scans. Such scans produce a series of structures that are typically calculated using some ground state method. Afterwards one may want to do additional or different calculations along the generated pathway such as excited state calculations or special property calculations. In this instance, the “multiple XYZ scan” feature is useful. If you request reading from a XYZ file via:

* xyzfile Charge Multiplicity FileName

this file could contain a number of structures. The format of the file is:

Number of atoms M
  Comment line
AtomName1 X Y Z
AtomName2 X Y Z
...
AtomNameM X Y Z
Number of atoms M
  Comment line
AtomName1 X Y Z
...

Thus, the structures are simply of the standard XYZ format and they are provided one after the other. There is no need to add any extra character between them. This was the case for ORCA versions older than 6.0.0, where the structures were separated by a “>” sign. The user can still use this format, if preferred, but is not needed anymore. After processing the XYZ file, single point calculations are performed on each structure in sequence and the results are collected at the end of the run in the same kind of trajectory.dat files as produced from trajectory calculations.

In order to aid in using this feature, the relaxed surface scans produce a file called MyJob.allxyz that is of the correct format to be re-read in a subsequent run.

4.2.6. Surface Scan Keywords

Table 4.4 %geom block input keywords and options for surface scanned geometry optimization.

Keyword

Option / Value

Description

Scan

B 1 2 = v1, v2, N

Scan atom 1 to atom 2 bond distance from v1 to v2 in N steps.

B 1 2 [v1 v2 ...]

Scan atom 1 to atom 2 bond with custom step values.

A 1 2 3

Scan an angle using atoms 1,2,3.

D 1 2 3 4

Scan a dihedral of atoms 1,2,3,4.

Simul_Scan

true

Enable simultaneous multidimensional scans (up to 3 coordinates).

FullScan

true

Forces full surface scan before TS optimization (!ScanTS).