<div dir="ltr"><p class="MsoNormal" style="margin-bottom:0.0001pt;direction:ltr"><span style="font-size:12pt;font-family:Arial,sans-serif">Dear Yu Hailin</span><span style="font-size:12pt;font-family:'Times New Roman',serif"></span></p>

<p class="MsoNormal" style="margin-bottom:0.0001pt;direction:ltr"><span style="font-size:12pt;font-family:Arial,sans-serif">In questions like this you should show
the SCF input that you use. The negative frequencies show that the crystal
is unstable or your calculations are not converged. It can be the numerical
parameters of the SCF calculation or the numerical parameters of the phonon
calculation. As far as I know, the only numerical parameter used in the phonon
calculation is</span><span class=""><span style="font-size:12pt;font-family:Arial,sans-serif;background-image:initial;background-repeat:initial"> </span></span><span style="font-size:12pt;font-family:Arial,sans-serif;background-image:initial;background-repeat:initial">tr2_ph</span>. </p>

<p class="MsoNormal" style="margin-bottom:0.0001pt;direction:ltr"><span style="font-size:12pt;font-family:Arial,sans-serif;background-image:initial;background-repeat:initial">If your </span><span style="font-size:12pt;font-family:Arial,sans-serif">calculation
is converged than the negative (imaginary) frequencies show that the structure
is unstable. You must do relaxation (probably to zero pressure) before the SCF
to ensure that the structure is on the minimum of the energy surface (at list
local minimum).</span></p>

<p class="MsoNormal" style="margin-bottom:0.0001pt;direction:ltr"><span style="font-size:12pt;font-family:Arial,sans-serif">Best regards</span></p>

<p class="MsoNormal" style="margin-bottom:0.0001pt;direction:ltr"><span style="font-size:12pt;font-family:Arial,sans-serif">Uri Argaman</span></p>

<p class="MsoNormal" style="margin-bottom:0.0001pt;direction:ltr"><span style="font-size:12pt;font-family:Arial,sans-serif">Ben-Gurion University </span></p>

<p class="MsoNormal" style="margin-bottom:0.0001pt;direction:ltr"><span style="font-size:12pt;font-family:Arial,sans-serif">Israel</span></p></div>