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<p>of the <a href="page.php?w=wave_function">wave function</a>.  Each component of the vibronic coupling vector can be calculated with <a href="page.php?w=numerical_differentiation">numerical differentiation</a> methods using wave functions at displaced geometries.  This is the procedure used in <a href="page.php?w=MOLPRO">MOLPRO</a>.</p>

<p>First order accuracy can be achieved with forward difference formula:</p>

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:</p>

<p>Second order accuracy can be achieved with central difference formula:</p>

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:</p>

<p>Here,  is a <a href="page.php?w=unit_vector">unit vector</a></p><p>
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