For small $x$, we can use the Taylor expansion $e^{-ax^2} \approx 1 - ax^2 + \frac{a^2x^4}{2} - ...$
Therefore, $V(x) \approx \frac{k}{2a}(1 - (1 - ax^2)) = \frac{k}{2}x^2$.
This is the potential for a simple harmonic oscillator with spring constant $k' = k$.
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