  We investigate the mass spectrum and electromagnetic transitions of
charmonia, emphasizing the instanton effects on them. The heavy-quark potential
consists of the Coulomb-like potential from one-gluon exchange and the linear
confining potential. We introduce the nonperturbative heavy-quark potential
derived from the instanton vacuum. We also consider the screened confining
potential, which better describes the electromagnetic decays of higher excited
states. Using this improved heavy-quark potential, we compute the mass spectrum
and electromagnetic decays of the charmonia. Focusing on the instanton effects,
we discuss the results compared with the experimental data and those from other
works. The instanton effects are marginal on the electromagnetic decays of
charmonia.
