# Example: (non-)Linear Transit: H2O¶

Download Transit_H2O.run

In ADF2008.01 a transit calculation option has been added in the new optimization branch. This is capable of performing both linear transits, and non-linear transits, and is the default when the LINEARTRANSIT or TRANSIT sub-block is included in the ‘Geometry’ block.

The new transit code works differently to the old: the transit is represented as a sequence of constrained optimizations. A ‘Constraints’ block is used to delineate the constraints applied at each stage of the transit.

Non-linear transits are possible, and can even be combined with linear transits in other coordinates. To perform a non-linear transit in a particular coordinate, explicit values must be given.

\$ADFBIN/adf << eor
Title WATER Transit (non-linear), with the new optimizer branch
Atoms
O             0.000000     0.000000     0.000000
H             0.000000    -0.689440    -0.578509
H             0.000000     0.689440    -0.578509
End
Symmetry NOSYM
Constraints
dist 1 2 0.8 1.0 1.25 1.5
angle 2 1 3 start=100.0 end=120.0
End
Basis
Type SZ
Core Large
End
Geometry
Transit 4
Optim Deloc
Converge 0.0001
End
End Input


In the example above, 4 values are given for the distance between atoms 1 and 2. This distance constraint will be applied simultaneously with the linear transit constraints for the angle, with other degrees of freedom optimized at each stage of the transit.

Finally, it should be pointed out that ‘partial constraints’ are used by default in the transit calculations. These constraints are not required to be fully met at each intermediate geometry, but are fully met at the converged geometries. You can use fully converged constraints by supplying the FULLCONVERGE option to the ‘Constraints’ subblock of the ‘Geometry’ block (not to be confused with the ‘Constraints’ block at root level).