The problem of a hydrogen atom in an impenetrable spherical cavity of small
radius R is considered on the basis of the first-order perturbational trea
tment of the electron-nucleus interaction taking explicit account of the bo
undary condition. An approximate analytic expression is derived for the ele
ctronic energy as a function of the shift of the nucleus off the centre. Th
e estimations obtained are in good qualitative and semiquantitative agreeme
nt with the results of the finite-difference calculations. Some general fea
tures of the energy dependences are discussed qualitatively.