  Instanton -- antiinstanton pair is considered as a source of singularity at
the Borel plane for the ground state energy of anharmonic oscillator. The
problem of defining the short range instanton -- antiinstanton interaction
reduces to calculation of a smooth part of the Borel function, which cannot be
found without explicit calculation of several terms of ordinary perturbation
theory. On the other hand, the large order terms of perturbative expansion are
dominated by large fluctuations in the functional integral like well separated
instanton and antiinstanton.
  The preasymptotics ($\sim 1/n$) of large order perturbation theory
contribution to the ground state energy of anharmonic oscillator was found
analytically. To this end the subleading long range asymptotics of the
classical instanton -- antiinstanton interaction, the one -- loop quantum
contribution to instanton -- antiinstanton interaction and the second quantum
correction to a single instanton density were considered.
