<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<style type="text/css" style="display:none;"> P {margin-top:0;margin-bottom:0;} </style>
</head>
<body dir="ltr">
<div class="elementToProof" style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 11pt; color: rgb(0, 0, 0);">
Hi Aswathy,</div>
<div class="elementToProof" style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 11pt; color: rgb(0, 0, 0);">
<br>
</div>
<div class="elementToProof" style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 11pt; color: rgb(0, 0, 0);">
I have run a fair few NEB calculations, both in QE and in other programs. Zero energy barrier can mean a few things, but most commonly it boils down to two main reasons.</div>
<div class="elementToProof" style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 11pt; color: rgb(0, 0, 0);">
<br>
</div>
<ol start="1" data-editing-info="{"applyListStyleFromLevel":false,"orderedStyleType":3}" style="margin-top: 0px; margin-bottom: 0px;">
<li style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 11pt; color: rgb(0, 0, 0); list-style-type: "1) ";">
<div class="elementToProof">The reactant and product geometries are not stationary points on the potential energy surface.</div>
</li><ol start="1" data-editing-info="{"applyListStyleFromLevel":true}" style="margin-top: 0px; margin-bottom: 0px; list-style-type: lower-alpha;">
<li style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 11pt; color: rgb(0, 0, 0);">
<div class="elementToProof">This can happens if the reactant and product are trapped in a local minimum, which nearly every quantum chemistry program struggles to deal with effectively.</div>
</li><li style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 11pt; color: rgb(0, 0, 0);">
<div class="elementToProof">Typically you can check if this is the case by examining the vibrational frequencies and see if there are imaginary/negative frequencies. Animating along those frequencies should push you into a more global minimum</div>
</li></ol>
<li style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 11pt; color: rgb(0, 0, 0); list-style-type: "2) ";">
<div class="elementToProof">The level of theory you are using could simply not be accurate enough to capture the process you wish to examine.</div>
</li><ol start="1" data-editing-info="{"applyListStyleFromLevel":true}" style="margin-top: 0px; margin-bottom: 0px; list-style-type: lower-alpha;">
<li style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 11pt; color: rgb(0, 0, 0);">
<div class="elementToProof">In my own experience, I know that when I measure single atom transfer in small molecules, the reaction barrier is only about 1-2 kcal/mol, which is extremely small and requires some very very precise calculations. Having used both
wB97M-V/def2-TZVPP on ORCA and vdW-DF-C09 with an ultrasoft pseudopotential, I have observed that the former is able to capture the barrier quite well, while the former predicts zero barrier almost every single time.</div>
</li></ol>
</ol>
<div class="elementToProof" style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 11pt; color: rgb(0, 0, 0);">
<br>
</div>
<div class="elementToProof" style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 11pt; color: rgb(0, 0, 0);">
I am sure that others will weigh in with their own thoughts and experiences, but I figured I would throw in my own just in case it helps. Also, as a side note, are the barriers you get exactly zero, or are they just very very close to zero, i.e. 10^-2 or 10^-3
kcal/mol? I've never encountered a barrier that is exactly zero, so if that is the case it may indicate something different.</div>
<div class="elementToProof" style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 11pt; color: rgb(0, 0, 0);">
<br>
</div>
<div class="elementToProof" style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 11pt; color: rgb(0, 0, 0);">
<br>
</div>
<div class="elementToProof" style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 11pt; color: rgb(0, 0, 0);">
Brock Dyer</div>
<div id="appendonsend" style="color: inherit;"></div>
<div class="elementToProof" style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 11pt; color: rgb(0, 0, 0);">
Ursinus College Department of Physics and Astronomy</div>
<div class="elementToProof" style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
Class of 2025<br>
</div>
<div class="elementToProof" style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<div class="elementToProof" style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<hr style="display: inline-block; width: 98%;">
<span style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);"><b>From:</b> users on behalf of ASWATHY JAYAPRAKASH<br>
<b>Sent:</b> Tuesday, March 11, 2025 4:26 AM<br>
<b>To:</b> users@lists.quantum-espresso.org<br>
<b>Subject:</b> [QE-users] Query Regarding Zero Activation Energy in NEB Calculations
</span>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<br>
</div>
<div style="direction: ltr;">Dear Quantum ESPRESSO Community,</div>
<div style="direction: ltr;"><br>
</div>
<p style="direction: ltr;">I am encountering an issue with NEB (Nudged Elastic Band) calculations. In some cases, I am getting
<b>zero activation energy</b> either for the forward reaction or the backward reaction. Could anyone please clarify what this means? Does this indicate a problem with my input files or calculation setup?</p>
<p style="direction: ltr;">I am following the same input structure that worked well in other cases, where I was able to obtain reasonable activation energies. However, in these particular cases, I consistently get zero barriers.</p>
<p style="direction: ltr;">Any insights or suggestions would be greatly appreciated.</p>
<p style="direction: ltr;">Thank you in advance for your help!</p>
<p style="direction: ltr;">Best regards,</p>
<p style="direction: ltr;">Aswathy Jayaprakash</p>
<p style="direction: ltr;">Research Scholar</p>
<p style="direction: ltr;">CUSAT</p>
</body>
</html>