<html><body><p>Dear Seongjae, </p><p><br></p><p> In addition to what Martin has written, I would like to point out that the difference between </p><p>the description of a crystal at 0 Kelvin and the same crystal at a room temperature </p><p>of 300 K corresponds to the difference between two </p><p>approximations in solid state physics (see e.g. the textbook </p><p>Neil Ashcroft and N. David Mermin, Solid State Physics): </p><p>For 0 K you use the static lattice approximation, </p><p>for 300 K you use the dynamic lattice approximation - it means include phonons </p><p>(that can be regarded as quantum units of lattice vibrations, summing them up you </p><p>get the whole vibration of your crystal). </p><p>Treating temperature dependent parameters (such as heat capacity) one usually </p><p>starts with the static lattice approximation and computing the ground state of </p><p>a sufficiently large supercell (using for example the code Wien2k) and </p><p>from there you get the second derivatives (Hessian) of the interatomic potentials </p><p>needed to construct the equations valid for the phonons (the dynamic matrix). </p><p>In real life this means that after finishing the calculations in Wien2k </p><p>(using the static lattice approximation) you continue with a code like is for example </p><p>PHONON by the group of prof. Krystof Parlinski and calculate the quantities you </p><p>like to know. </p><p>(see http://www.computingformaterials.com/index.html and others, see </p><p>http://www.wien2k.at/reg_user/unsupported/ after the item Phonon). </p><p>Best regards </p><p>Tomas </p><p><br></p><p><br></p><blockquote><div><p>Dear group, </p><p><br></p><p>As an engineering researcher with great lack in understanding the ab initio calculations, </p><p>I have basically believed that the first-principle calculation results demonatrate rather </p><p>"ideal" values presumably obtained at "0 K" and they need to be adjusted by proper mathematical </p><p>models formulated as a function of temperature for reachiing the more practical values at non-0 K values. </p><p><br></p><p>However, in many pieces of literature, they are trying to compare the ab initio calculation </p><p>results and the measurement results at non-0 K, particularly at room temperature. </p><p><br></p><p>I'm wondering what sort of foundation is required for believing that the simulation results </p><p>can be treated as those obtained at 300 K. In other words, what models or equations can be </p><p>adopted for taking the exact band structures and related parameters (Eg, effective mass, etc.) </p><p>in hand in performing the first-principle simulations? </p><p><br></p><p>It will be appreciated if you fix my fault and share some wisdom. Many thanks. </p><p><br></p><p>- Sincerely, Seongjae. </p><p><br></p>
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