<HTML><BODY style="word-wrap: break-word; -khtml-nbsp-mode: space; -khtml-line-break: after-white-space; "><BR><DIV><DIV>On Apr 29, 2007, at 11:31 PM, li niu wrote:</DIV><BR class="Apple-interchange-newline"><BLOCKQUOTE type="cite"><DIV>Dear Paolo U,</DIV> <DIV> </DIV> <DIV>Thanks for your reply.I'm very interested for your finite electric field method.<BR>The dielectric polarizability tensor can be calculated by taking dinite differences of atomic forces.<BR></DIV></BLOCKQUOTE><DIV><BR class="khtml-block-placeholder"></DIV>NO: forces have nothing to do with the dielectric tensor. If you want to calculate the dielectric constant using finite electric fields (why? linear response is there exactly to avoid that), you will have to calculate finite differences of electric polarizations corresponding to different diractions of the applied field.</DIV><DIV><BR><BLOCKQUOTE type="cite"><DIV>I have several questions to ask you.</DIV> <DIV><BR></DIV></BLOCKQUOTE><DIV><BR class="khtml-block-placeholder"></DIV>...</DIV><DIV><BR><BLOCKQUOTE type="cite"><DIV>2. For the calculation of the off-diagonal mixedterms, two different electric fields are required. This need apply electric fields of intensity - 2h/1.414 , </DIV> <DIV>-h/1.414, 0, h/1.414 and 2h/1.414 along the three different couples of cartesian directions, and apply equations.<BR>But how can I apply simultaneously finite fields along two different Cartesian directions in cp.x code?[ESSPRESSO-3.2]<BR></DIV></BLOCKQUOTE><DIV><BR class="khtml-block-placeholder"></DIV>here, I can hardly follow you. Two finite (or infinitesimal, for what matters) fileds applied along different directions make ONE electrc field directed along the the sum of the two. Aren't electric fields VECTORS?</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>SB</DIV><BR><DIV> <SPAN class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 14px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><SPAN class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 14px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; 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