<div dir="ltr">  Dear Pavel,<br>Thank for your reply,<div>In the dielectric function expression the wave vector of light q is much smaller than any typical wave vector of electrons in the system, it can be evaluated for small  q by k · p perturbation theory.(check this reference please ;<span style="font-size:13px;font-family:Arial,sans-serif">AMBROSCH-DRAXL, Claudia et SOFO, Jorge O. Linear optical properties of solids within the full-potential linearized augmented planewave method. </span><i style="font-size:13px;font-family:Arial,sans-serif">Computer physics communications</i><span style="font-size:13px;font-family:Arial,sans-serif">, 2006, vol. 175, no 1, p. 1-14.)</span></div><div>I want know these values of wave vector of light q .</div><div>Best regards</div><div>Siham</div><br><br><br><div id="mt-signature">
        <table border="0" cellpadding="8" cellspacing="0" style="user-select: none;">
            <tbody><tr>
                <td>
                    <a href="https://mailtrack.io?utm_source=gmail&utm_medium=signature&utm_campaign=signaturevirality5&" class="" style="text-decoration:none">
                        <img src="https://s3.amazonaws.com/mailtrack-signature/sender_notified.gif" alt="Mailtrack" class="" width="32" height="32">
                    </a>
                </td>
                <td>
                    <span style="color:#777">Sender notified by</span> <br>
                    <a href="https://mailtrack.io?utm_source=gmail&utm_medium=signature&utm_campaign=signaturevirality5&" class="mt-install" style="color:#4374f7">Mailtrack</a>
                    <span style="color:transparent;font-size:0">28/01/20 à 21:36:06</span>
                </td>
                <td>
                    
                </td>
            </tr>
        </tbody></table>
    </div><img width="0" height="0" class="mailtrack-img" alt="" style="display:flex" src="https://mailtrack.io/trace/mail/1861dc90bbb8c8320517481d856dc8ff3e0ca934.png?u=2821519"></div>