In spontaneous symmetry breaking discussion in Weinberg Chapter 19 section 19.1, he says that the off-diagonal elements between two vacua (|VAC,+>±|VAC−>) is suppressed by the factor of exp(−CV). For a scalar field (−12(∂ϕ)2−V(ϕ)) theory by analogy to wave mechanical problem of barrier penetration we have C=∫ˉϕ−ˉϕ√2V(ϕ)dϕ. I am trying to understand how to get this. Please let me know if you know how to obtain this mathematically.