Consider the equation (0.1) ut = ∆u− V u+ au p in R × (0, T ); u(x, 0) = φ(x) 0, in R, where p > 1, n ≥ 2, T ∈ (0,∞], V (x) ∼ ω |x| as |x| → ∞, for some ω 6= 0, and a(x) is on the order |x| as |x| → ∞, for some m ∈ (−∞,∞). A solution to the above equation is called global if T = ∞. Under some additional technical conditions, we calculate a critical exponent p such that global solutions exist fo...