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<div class="section" id="excited-states">
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<span id="id1"></span><h1>Excited states<a class="headerlink" href="#excited-states" title="Permalink to this headline">¶</a></h1>
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<p>It is possible to run excited states calculations with the quantum
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package. To do this, set <a class="reference internal" href="../modules/determinants.html#cmdoption-determinants-arg-n-states"><code class="xref std std-option docutils literal notranslate"><span class="pre">determinants</span> <span class="pre">n_states</span></code></a> to the number
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of requested states. The selection criterion will be the maximum of the
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selection criteria for each state. If the Davidson diagonalization has
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difficulties to converge, increase the <a class="reference internal" href="../modules/davidson.html#cmdoption-davidson-arg-n-states-diag"><code class="xref std std-option docutils literal notranslate"><span class="pre">davidson</span> <span class="pre">n_states_diag</span></code></a>
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value.</p>
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<p>When computing multiple states, it is good to have the
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<a class="reference internal" href="../modules/determinants.html#cmdoption-determinants-arg-s2-eig"><code class="xref std std-option docutils literal notranslate"><span class="pre">determinants</span> <span class="pre">s2_eig</span></code></a> flag <code class="docutils literal notranslate"><span class="pre">true</span></code>. This will force the Davidson
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algorithm to choose only vectors with a value of <span class="math notranslate nohighlight">\(\widehat{S^2}\)</span> equal to
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<a class="reference internal" href="../modules/determinants.html#cmdoption-determinants-arg-expected-s2"><code class="xref std std-option docutils literal notranslate"><span class="pre">determinants</span> <span class="pre">expected_s2</span></code></a>. Otherwise, different spin states
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will come out in the diagonalization.</p>
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<p>The <em>Quantum Package</em> doesn’t take account of the symmetry. Due to numerical noise,
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excited states of different symmetries may enter in the calculation.
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Note that it is possible to make state-average calculation of states
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with different symmetries and/or different spin multiplicities.</p>
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<p>To include excited state of all possible symmetries, a simple trick is
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to run a preliminary multi-state <abbr title="Configuration Interaction with Single Excitations">CIS</abbr> calculation using the <a class="reference internal" href="../programs/cis.html#cis"><span class="std std-ref">cis</span></a>
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program, and then running the selected <abbr title="Full Configuration Interaction">FCI</abbr> restarting from the <abbr title="Configuration Interaction with Single Excitations">CIS</abbr>
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states, setting <a class="reference internal" href="../modules/determinants.html#cmdoption-determinants-arg-read-wf"><code class="xref std std-option docutils literal notranslate"><span class="pre">determinants</span> <span class="pre">read_wf</span></code></a> to <code class="docutils literal notranslate"><span class="pre">true</span></code>.</p>
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<p>Usually, it is good practice to use state-averaged natural <abbr title="Molecular Orbitals">MOs</abbr> so that
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all states have <abbr title="Molecular Orbitals">MOs</abbr> of comparable quality. This allows for a faster
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convergence of excitation energies.</p>
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<div class="admonition seealso">
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<p class="first admonition-title">See also</p>
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<p class="last">The documentation of the <code class="xref c c-func docutils literal notranslate"><span class="pre">scf()</span></code>, <code class="xref c c-func docutils literal notranslate"><span class="pre">cis()</span></code> and
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<code class="xref c c-func docutils literal notranslate"><span class="pre">fci()</span></code> programs.</p>
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