Contents: Table of Contents   PUNCH directives  Chapter 5  Isotope Effects

Transition States

Calculating force constants and vibrational frequencies at a transition state uses exactly the same commands i.e. SECDER or FORCE, as are used at an equilibrium geometry. The only difference is in the interpretation of the results. For example if the following dataset,

TITLE
O2...HF  LINEAR   MP2/6-31G* FREQUENCIES

NOTE a large UHF MP2 force constant run on a dimer
NOTE this is a stationary geometry but it is not
NOTE a minimum

MULTIPLICITY 3
ANGSTROMS
ATOMS
O    8    0.000000    0.000000   -2.067433
LIBRARY O631G*
END
O    8    0.000000    0.000000   -0.823718
LIBRARY O631G*
END
H    1    0.000000    0.000000    1.472793
LIBRARY H631G
END
F    9    0.000000    0.000000    2.406268
LIBRARY F631G*
END
END
NOPRINT OCCVECTORS
UHF
NPOINTS 2
MP2
FORCE
START
FINISH

was run you would find that instead of all the vibrational frequencies being zero or positive, two have imaginary values (these are actually displayed as negative numbers i.e. the program prints -89 not 89i in the example mentioned). The imaginary frequencies indicate a negative force constant i.e. a curvature corresponding to a saddle point.