  A new, more comprehensive model of gas-grain chemistry in hot molecular cores
is presented, in which nondiffusive reaction processes on dust-grain surfaces
and in ice mantles are implemented alongside traditional diffusive
surface/bulk-ice chemistry. We build on our nondiffusive treatments used for
chemistry in cold sources, adopting a standard collapse/warm-up physical model
for hot cores. A number of other new chemical model inputs and treatments are
also explored in depth, culminating in a final model that demonstrates
excellent agreement with gas-phase observational abundances for many molecules,
including some (e.g. methoxymethanol) that could not be reproduced by
conventional diffusive mechanisms. Observed ratios of structural isomers methyl
formate, glycolaldehyde and acetic acid are well reproduced by the models. The
main temperature regimes are identified in which various complex organic
molecules (COMs) are formed. Nondiffusive chemistry advances the production of
many COMs to much earlier times and lower temperatures than in previous model
implementations. Those species may form either as by-products of simple-ice
production, or via early photochemistry within the ices while external UV
photons can still penetrate. Cosmic ray-induced photochemistry is less
important than in past models, although it affects some species strongly over
long timescales. Another production regime occurs during the high-temperature
desorption of solid water, whereby radicals trapped in the ice are released
onto the grain/ice surface, where they rapidly react. Several recently-proposed
gas-phase COM-production mechanisms are also introduced, but they rarely
dominate. New surface/ice reactions involving CH and CH$_2$ are found to
contribute substantially to the formation of certain COMs.
