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<!doctype html><html><head> <meta charset="utf-8"> <meta name="keywords" content=""> <meta name="description" content=""> <meta name="viewport" content="width=device-width, initial-scale=1.0, viewport-fit=cover"><meta name="robots" content="index, follow" /> <link rel="shortcut icon" type="image/png" href="favicon.png"> <link rel="stylesheet" type="text/css" href="./css/bootstrap.min.css?9876"><link rel="stylesheet" type="text/css" href="style.css?1620"><link rel="stylesheet" type="text/css" href="./css/animate.min.css?1042"> <title>Home</title> <!-- Analytics --> <!-- Analytics END --> </head><body><!-- Preloader --><div id="page-loading-blocs-notifaction" class="page-preloader"></div><!-- Preloader END --><!-- Main container --><div class="page-container"> <!-- bloc-0 --><div class="bloc none bgc-princeton-orange full-width-bloc" id="bloc-0"><div class="container bloc-sm bloc-no-padding-lg"><div class="row"><div class="col"><nav class="navbar navbar-light row navbar-expand-md" role="navigation"><a class="navbar-brand" href="index.html"><img src="img/logo.png" alt="logo" /></a><button id="nav-toggle" type="button" class="ml-auto ui-navbar-toggler navbar-toggler border-0 p-0" data-toggle="collapse" data-target=".navbar-21982" aria-expanded="false" aria-label="Toggle navigation"><span class="navbar-toggler-icon"></span></button><div class="navbar-content-area col"><h5 class="mg-lg float-lg-none h5-style tc-white">Indian Institute of Information Technology-Allahabad<br>भारतीय सूचना प्रौद्योगिकी संस्थान-इलाहाबाद<br></h5></div></nav></div></div></div></div><!-- bloc-0 END --><!-- bloc-1 --><div class="bloc l-bloc none" id="bloc-1"><div class="container bloc-sm bloc-no-padding-lg"><div class="row"><div class="col"><nav class="navbar row navbar-light navbar-expand-md" role="navigation"><button id="nav-toggle" type="button" class="ml-auto ui-navbar-toggler navbar-toggler border-0 p-0" data-toggle="collapse" data-target=".navbar-16243" aria-expanded="false" aria-label="Toggle navigation"><span class="navbar-toggler-icon"></span></button><div class="collapse navbar-collapse navbar-16243 col-lg-12"><ul class="site-navigation nav navbar-nav ml-auto"><li class="nav-item"><a href="index.html" class="nav-link ltc-black">Home</a></li><li class="nav-item"><a href="news.html" class="nav-link a-btn ltc-black">News</a></li><li class="nav-item"><a href="highlights.html" class="nav-link a-btn ltc-black">Highlights</a></li><li class="nav-item"><a href="people.html" class="nav-link a-btn ltc-black">People</a></li><li class="nav-item"><a href="publications_new.html" class="nav-link a-btn ltc-black">Publications</a></li><li class="nav-item"><a href="projects.html" class="nav-link a-btn ltc-black">Projects</a></li><li class="nav-item"><a href="research.html" class="nav-link a-btn ltc-black">Research</a></li><li class="nav-item"><a href="teaching.html" class="nav-link a-btn ltc-black">Teaching</a></li><li class="nav-item"><a href="online-tutorials.html" class="nav-link a-btn ltc-black">Online Tutorials</a></li><li class="nav-item"><a href="learning-resources.html" class="nav-link a-btn ltc-black">Learning Resourses</a></li><li class="nav-item"><a href="facilities.html" class="nav-link a-btn ltc-black">Facilities</a></li><li class="nav-item"><a href="outreach.html" class="nav-link a-btn ltc-black">Outreach</a></li></ul></div></nav></div></div></div></div><!-- bloc-1 END --><!-- bloc-2 --><div class="bloc bgc-dark-midnight-blue full-width-bloc" id="bloc-2"><div class="container bloc-lg bloc-no-padding-lg"><div class="row"><div class="col"><h4 class="mg-md animDelay04 none animLoopInfinite h4-news--style animSpeedSlow animated fadeIn text-justify tc-white-2" data-appear-anim-style="fadeIn">News: Opening for SERB CRG JRF position. Click <a class="ltc-electric-yellow" href="downloads/Advt_JRF_sitangshu.pdf" target="_blank">here</a> from more information. Regular Ph.D. positions are advertised. Click <a class="ltc-electric-yellow" href="https://www.iiita.ac.in/downloads/announcements/uploads/FINAL%20Advertisement%20for%20PG_PhD%20Admission%20(2)891.pdf" target="_blank">here</a> for more details. </h4></div></div></div></div><!-- bloc-2 END --><!-- bloc-3 --><div class="bloc" id="bloc-3"><div class="container bloc-lg bloc-no-padding-lg"><div class="row"><div class="col"><h1 class="h1-1-style text-justify mg-md tc-dark-magenta"><strong>Update</strong>: I have tried to automatize the open source ab-initio codes Quantum Espresso and YAMBO to compute ground state and excited state energies. These automatic scripts using Quantum Espresso (6.7MAX) +YAMBO (v5.1) are now available in my <a class="ltc-ferrari-red" href="https://github.com/SitangshuIIITA/Automatic-Quantum-Espresso-to-YAMBO-scripts-in-HPC" target="_blank">GitHub</a> I have tried to write the flow in a super easy to understand format, so that anyone can run these lengthy and cascaded consecutive flows without losing much time in between. The folder names are compatible also to the yambopy process-flow.</h1></div></div></div></div><!-- bloc-3 END --><!-- bloc-1 --><div class="bloc tc-black none full-width-bloc" id="bloc-1"><div class="container bloc-lg bloc-no-padding-lg"><div class="row row-style"><div class="col-lg-12"><h4 class="mg-md text-lg-center mx-auto d-block tc-black">Electronic Structure Theory Group<br></h4></div><div class="col-12 order-lg-0 col-lg-8 offset-lg-2"><div class="text-center"><div id="carousel-28189" class="carousel slide" data-ride="carousel" data-interval="8000"><ul class="carousel-indicators"><li data-target="#carousel-28189" data-slide-to="0" class="active"></li><li data-target="#carousel-28189" data-slide-to="1"></li><li data-target="#carousel-28189" data-slide-to="2"></li><li data-target="#carousel-28189" data-slide-to="3"></li><li data-target="#carousel-28189" data-slide-to="4"></li></ul><div class="carousel-inner" role="listbox"><div class="carousel-item active"><img class="d-inline-block w-100" alt="Excion imaging in h-BN " src="img/Home_fig_1.png" /><div class="carousel-caption"><h4 class="p-bloc-1-style text-lg-center btn-resize-mode mx-auto d-block mg-clear tc-black">Exciton wavefunction imaging in h-BN. The black dot is the hole on N-atom (Phys. Rev. B, 99, 165201, (2019)).</h4></div></div><div class="carousel-item"><img class="d-inline-block w-100" alt="placeholder image-wide" src="img/exct_NP.png" /><div class="carousel-caption"><h4 class="h4-3-style mg-sm tc-black">Most important bound excitons in NP (group V-V) monolayer (ACS J. Phys. Chem. C, 125, 12738, (2021).</h4></div></div><div class="carousel-item"><img class="d-inline-block w-100" alt="TOC" src="img/TOC.png" /><div class="carousel-caption"><h4 class="h4-4-style mx-auto d-block text-lg-center mg-clear tc-black">WSe2 exciton spin forbidden transition in presece of lattice vibrations (Phys. Rev. B, 98, 045143, (2018)). </h4></div></div><div class="carousel-item"><img class="d-inline-block w-100" alt="gaas_shg" src="img/gaas_shg.png" /><div class="carousel-caption"><h4 class="h4-5-bloc-1-style mg-clear tc-black">Second harmonic coefficient and 2ω resonances in GaAs monolayer. Symbols are from bulk GaAs (Phys. Rev. B 101, 155132, (2020)). </h4></div></div><div class="carousel-item"><img class="d-inline-block w-100" alt="gaas_kramer" src="img/ri.png" /><div class="carousel-caption"><h4 class="h4-6-style mx-auto d-block mg-sm tc-black">Refractive index and absorbance in NP (group V-V) monolayer from Bethe-Salpeter Equation (ACS J. Phys. Chem. C, 125, 12738, (2021))</h4></div></div></div><a class="carousel-nav-controls carousel-control-prev" href="#carousel-28189" role="button" data-slide="prev"><svg width="26" height="26" viewBox="0 0 32 32"><path class="carousel-nav-icon carousel-prev-icon" d="M22,2L9,16,22,30"></path></svg><span class="sr-only">Previous</span></a><a class="carousel-nav-controls carousel-control-next" href="#carousel-28189" role="button" data-slide="next"><svg width="26" height="26" viewBox="0 0 32 32"><path class="carousel-nav-icon carousel-next-icon" d="M10.344,2l13,14-13,14"></path></svg><span class="sr-only">Next</span></a></div></div></div><div class="col-12"><h2 class="mg-md h2-style text-lg-center tc-ferrari-red"><strong><br><br><br>Ab-initio/First principles calculations in two-dimensional materials</strong><br></h2><p class="text-justify none mg-md"><br>Electronic structure theory is the study of how electrons in a crystal interacts with atomic vibrations, light, defects and other external excitations via charges and energies that builds exciting electronic, thermal, magnetic and optical properties. In 1965, a series of two papers by Walter Kohn (Austrian born, American scientist) together with Pierre Hohenberg (French born American physicist) and Lu Jeu Sham (Chinese physicist) and later with John Pople (Bristish chemist) demonstrated that the their ground-state energy functional theory, the so-called “density functional theory” (sometimes also called as “ab-initio formalism” or “first principles theory), is inevitable to properly describe the electronic affairs inside crystal and attracted a Noble prize in chemistry (1998), shared between Kohn and Pople.<br>In early days, their theory was quite a challenge to solve because of lack of obvious computational resources, thanks to the now commercially available hardware accelerators and the high-speed data communicators, their equations are now both mathematically enjoyable to solve and develop.<br>In this group, we study the electronic correlations using the most advanced linear response many body perturbation theory. Such schemes are beyond the standard ground state energy functional theory and arrows to a more advanced Hartree-Fock theory, Random-phase approximation, self-consistent GW approximation and Bethe-Salpeter equation. These challenging schemes are computational-costly practice, but if mastered, can be done systematically and elegantly.<br></p></div></div></div></div><!-- bloc-1 END --><!-- bloc-5 --><div class="bloc l-bloc" id="bloc-5"><div class="container bloc-lg"><div class="row"><div class="col"></div></div></div></div><!-- bloc-5 END --><!-- bloc-5 --><div class="bloc l-bloc" id="bloc-5"><div class="container bloc-lg bloc-sm-lg"><div class="row"><div class="col"><div ><div id="sfcgj52a7pfl2y2bx4f998s6becz8wq6uh1"></div><script type="text/javascript" async="" src="https://counter2.stat.ovh/private/counter.js?c=gj52a7pfl2y2bx4f998s6becz8wq6uh1&down=async"></script><noscript>&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;a href="https://www.freecounterstat.com" title="website counter"&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;img src="img/lazyload-ph.png" data-src="https://counter2.stat.ovh/private/freecounterstat.php?c=gj52a7pfl2y2bx4f998s6becz8wq6uh1" border="0" title="website counter" alt="website counter"&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;/a&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;</noscript></div></div></div></div></div><!-- bloc-5 END --><!-- ScrollToTop Button --><a class="bloc-button btn btn-d scrollToTop" onclick="scrollToTarget('1',this)"><svg xmlns="http://www.w3.org/2000/svg" width="22" height="22" viewBox="0 0 32 32"><path class="scroll-to-top-btn-icon" d="M30,22.656l-14-13-14,13"/></svg></a><!-- ScrollToTop Button END--><!-- bloc-4 --><div class="bloc bgc-black l-bloc" id="bloc-4"><div class="container bloc-sm-lg"><div class="row bgc-black"><div class="col bgc-black"><h1 class="mg-md h1-style text-lg-center tc-electric-yellow">Website developed by Dr. Sitangshu Bhattacharya, ECE, IIITA (<i>sitangshu</i>[at]iiita.ac.in). 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