In the world of Density Functional Theory (DFT), the pseudopotential is perhaps the most critical "black box" a researcher must master. If you are using Quantum Espresso (QE), choosing the right pseudopotential is the difference between an accurate simulation and a meaningless calculation.<h2 id="033a58bf-1f73-4633-89e8-a025895961c7" data-toc-id="033a58bf-1f73-4633-89e8-a025895961c7" style="font-size: 24px; line-height: 1.2;">What is a Pseudopotential?</h2>In an atom, electrons are divided into two groups: core electrons and valence electrons.<ul style="padding-inline-start: 32px; font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;"><li style="font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;">Core electrons are tightly bound to the nucleus and barely change during chemical bonding.</li><li style="font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;">Valence electrons participate in bonding and determine the physical properties of the material.</li></ul>A pseudopotential is an effective potential that replaces the complicated effects of the motion of core electrons and the nucleus with a simpler "pseudo" potential. This allows us to only treat the valence electrons explicitly, drastically reducing computational cost.<hr><h2 id="8105f16e-7daf-4f82-bfe9-00a2c495e1f9" data-toc-id="8105f16e-7daf-4f82-bfe9-00a2c495e1f9" style="font-size: 24px; line-height: 1.2;">The Three Main Types of Pseudopotentials in QE</h2>Quantum Espresso supports several functional forms of pseudopotentials. You will usually find these in the <code>.UPF</code> (Unified Pseudopotential Format) file.<h3 id="0670f7ea-9a15-4a11-be3d-52fb72dc5075" data-toc-id="0670f7ea-9a15-4a11-be3d-52fb72dc5075" style="font-size: 20px; line-height: 1.2;">1. Norm-Conserving (NC)</h3><ul style="padding-inline-start: 32px; font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;"><li style="font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;">How it works: Outside a certain "cutoff radius" ($r_c$), the pseudo-wavefunction is identical to the true all-electron wavefunction.</li><li style="font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;">Pros: Highly accurate and reliable for a wide range of systems.</li><li style="font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;">Cons: Requires a very high plane-wave energy cutoff (<code>ecutwfc</code>), making calculations slow.</li></ul><h3 id="758781de-be24-4c95-ad56-ab442b6b2b84" data-toc-id="758781de-be24-4c95-ad56-ab442b6b2b84" style="font-size: 20px; line-height: 1.2;">2. Ultrasoft (USPP)</h3><ul style="padding-inline-start: 32px; font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;"><li style="font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;">How it works: Developed by David Vanderbilt, these relax the "norm-conserving" constraint.</li><li style="font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;">Pros: Allows for a much lower <code>ecutwfc</code>, significantly speeding up calculations.</li><li style="font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;">Cons: Requires an additional "charge density" cutoff (<code>ecutrho</code>), typically 8–10 times the <code>ecutwfc</code>.</li></ul><h3 id="456fcb50-8c7f-49ff-9f70-cf8c150f19ba" data-toc-id="456fcb50-8c7f-49ff-9f70-cf8c150f19ba" style="font-size: 20px; line-height: 1.2;">3. Projector Augmented Wave (PAW)</h3><ul style="padding-inline-start: 32px; font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;"><li style="font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;">How it works: Considered the "gold standard," it combines the efficiency of Ultrasoft with the accuracy of All-Electron methods.</li><li style="font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;">Pros: Excellent for properties involving the nucleus (like NMR or hyperfine splitting).</li><li style="font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;">Cons: Computationally slightly heavier than USPP but more robust.</li></ul><hr><h2 id="a89b600f-9328-42ac-9ca3-8339a52b47fa" data-toc-id="a89b600f-9328-42ac-9ca3-8339a52b47fa" style="font-size: 24px; line-height: 1.2;">How to Read a Pseudopotential Filename</h2>Quantum Espresso filenames are descriptive. For example: <code>Fe.pbe-n-kjpaw_psl.1.0.0.UPF</code><ul style="padding-inline-start: 32px; font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;"><li style="font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;">Fe: The chemical symbol (Iron).</li><li style="font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;">pbe: The Exchange-Correlation functional (Percew-Burke-Ernzerhof).</li><li style="font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;">kjpaw: Indicates it is a PAW pseudopotential.</li><li style="font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;">psl: From the "PSlibrary" collection.</li></ul><hr><h2 id="3b206f18-eaa5-40a0-9c59-bf128f3ec8fd" data-toc-id="3b206f18-eaa5-40a0-9c59-bf128f3ec8fd" style="font-size: 24px; line-height: 1.2;">Implementing Pseudopotentials in your Input File</h2>In your <code>pw.x</code> input file, you must define where the potentials are located and which file belongs to which element.Fortran<pre><code>&amp;CONTROL
 pseudo_dir = './pseudo/',
/
&amp;SYSTEM
 ecutwfc = 40.0,
 ecutrho = 320.0,
/
ATOMIC_SPECIES
 Si 28.085 Si.pbe-n-rrkjus_psl.1.0.0.UPF</code></pre><blockquote style="font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;">Expert Tip: If you use Ultrasoft or PAW, always set <code>ecutrho</code> to at least 8x or 12x the value of <code>ecutwfc</code>. For Norm-Conserving, 4x is usually sufficient.</blockquote><hr><h2 id="5d5a3a3d-dde9-4c8a-a5fe-981ef1452550" data-toc-id="5d5a3a3d-dde9-4c8a-a5fe-981ef1452550" style="font-size: 24px; line-height: 1.2;">Frequently Asked Questions (FAQ)</h2><h3 id="e7ab602e-49a8-451e-b6c8-0e851a57ad1b" data-toc-id="e7ab602e-49a8-451e-b6c8-0e851a57ad1b" style="font-size: 20px; line-height: 1.2;">Q1: Where can I download reliable pseudopotentials?</h3>The best sources for Quantum Espresso are:<ol style="padding-inline-start: 32px; font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;"><li style="font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;">SSSP (Standard Solid-State Pseudopotentials): Highly recommended for most materials.</li><li style="font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;">PSlibrary: A massive, high-quality collection integrated with QE.</li><li style="font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;">PseudoDojo: Excellent for norm-conserving potentials.</li></ol><h3 id="146c654e-05b6-4617-a49c-ef3bac04e78f" data-toc-id="146c654e-05b6-4617-a49c-ef3bac04e78f" style="font-size: 20px; line-height: 1.2;">Q2: Can I mix different types of pseudopotentials?</h3>No. You should never mix functionals. If your Silicon potential is <code>PBE</code>, your Oxygen potential must also be <code>PBE</code>. Mixing LDA and PBE will lead to unphysical results.<h3 id="f0fee6bb-5167-47a9-b8eb-fd5c5ef85595" data-toc-id="f0fee6bb-5167-47a9-b8eb-fd5c5ef85595" style="font-size: 20px; line-height: 1.2;">Q3: How do I know if my pseudopotential is "good"?</h3>You must perform a convergence test. Plot Total Energy vs. <code>ecutwfc</code>. The energy should flatten out as the cutoff increases. If it doesn't converge, your potential may be "too hard."<hr><h2 id="95eb733a-5e10-4da3-a5da-31ee4b0a50c1" data-toc-id="95eb733a-5e10-4da3-a5da-31ee4b0a50c1" style="font-size: 24px; line-height: 1.2;">Citations &amp; Resources</h2><ul style="padding-inline-start: 32px; font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;"><li style="font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;">Giannozzi, P., et al. (2009). "QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials."</li><li style="font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;">Vanderbilt, D. (1990). "Soft self-consistent pseudopotentials in a generalized eigenvalue formalism." Physical Review B.</li><li style="font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;"><a target="_blank" rel="noopener" class="ng-star-inserted" href="https://www.quantum-espresso.org/pseudopotentials/" style="font-family: &quot;Google Sans Text&quot;, sans-serif !important; line-height: 1.15 !important; margin-top: 0px !important;">Quantum Espresso Official Pseudopotential Page</a></li></ul>

Master pseudopotentials in Quantum Espresso. Learn about NC, USPP, and PAW types, naming conventions, and implementation tips for accurate DFT calculations.

What is Pseudopotentials in Quantum Espresso: A Comprehensive Guide

The Three Main Types of Pseudopotentials in QE

Implementing Pseudopotentials in your Input File

Q1: Where can I download reliable pseudopotentials?

Q2: Can I mix different types of pseudopotentials?

Q3: How do I know if my pseudopotential is "good"?

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What is Pseudopotentials in Quantum Espresso: A Comprehensive Guide

What is Pseudopotentials in Quantum Espresso: A Comprehensive Guide

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