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Specifying a Scan: Step Count and Step Size

A scan is fully defined by three numbers: a starting value \(q_0\), a step size \(\Delta q\), and a number of steps \(N\). Gaussian generates the scan grid as

\[ q_n = q_0 + n\,\Delta q, \qquad n = 0, 1, \dots, N \]

giving \(N+1\) total scan points. In this tutorial's input,

B3   0.90 s 10 0.02

corresponds to \(q_0 = 0.90\) Å, \(\Delta q = 0.02\) Å, and \(N = 10\), producing 11 points:

\[ q_n \in \{0.90,\ 0.92,\ 0.94,\ \dots,\ 1.10\}\ \text{Å} \]

The same three numbers appear in the modredundant scan syntax (S nsteps stepsize after a B, A, or D coordinate specification) — only the surrounding input format differs, not the underlying grid construction.

A finer step size resolves the shape of the curve more precisely but multiplies the number of full optimizations required; a coarser step size is cheaper but risks missing a sharp feature — such as a narrow barrier — between two sampled points.