Contents: Table of Contents   Transition States  Chapter 5  Numerical force constant calculations

Isotope Effects

By default the vibrational analysis is performed using the masses of the commonest isotopes for each nuclear species. The cartesian force constant matrix is of cource independent of mass so if you wish to study the effect of isotopic substitution on the harmonic frequencies this can be done just by repeating the final analysis. This requires the WEIGHTS and INTENSITY keywords. Here is an example. This calculation on ethylene oxide at the equilibrium geometry. First the force constants are calculated and analysed using the default atomic masses. The WEIGHTS command is then used to change the masses of two of the atoms and the INTENSITY command causes the vibrational analyses to be repeated using the new masses. In the format of the WEIGHTS command note that the atoms are refered to by number rather than by name. This means that if there are several atoms of the same type you can change the masses of just some of these. For example in the present case the substitution gives the chiral form of C₂H₂D₂O. Those atoms not mentioned in a WEIGHTS command have their default masses, or retain the setting giving by a previous command.

TITLE
ETHYLENE OXIDE 6-31G** 
SYMMETRY
CNV  2
END
BASIS 631G**
ATOMS
OXYGEN    8    0            0               1.5612790922
CARBON    6    0            1.3725147847   -0.7037349187
HYDROGEN  1    1.7195922714 2.3852633821   -1.1079616887
END
SECDER
START

WEIGHTS
4 2.0142
7 2.0142
END

INTENSITY
START
FINISH