For most reference data, the default weights are suitable. If a reason exists to increase or decrease the weight of a particular property for a particular reference data file, then the weights can be changed by using these keywords.
All four keywords behave in the same manner: the default weight is multiplied by n.n. n.nn should be in the range 0.1 to 10. The effect of changing a weight is proportional to the square of the weight (because the function being minimized is the root-mean-square).
Reasons for changing the default weights
The compound is important. Benzene is the archetypical aromatic compound, so it is more important than most other compounds. For this reason its heat of formation would be given a higher weight, e.g., HWT=2.0.
The accuracy is low. Carbon suboxide, C3O2, is not very important, and the only reference, Cox and Pilcher, is already quite old. So its heat of formation weight is set to HWT=0.5.
The interaction is weak. Weak interactions are, of their nature, small, so a higher accuracy is important. The accuracy can be increased by increasing the weight of the heat of formation or the geometry. A good example is provided by the water dimer interaction energy and geometry. For the different water dimers, the geometry and heat of formation weight are both increased by very large amounts.
Default weights should not be changed for organic or inorganic compounds. Because having different weights for different families of compounds is important, this is done automatically. The relative default weights are:
Compounds of H, C, N, and O, only: 1.0
Compounds with other elements important in biochemistry: 0.9
Compounds with other main-group elements: 0.8
Compounds involving transition metals: 0.7