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the pressure calibration of 50 GPa at 900 GPa. The devia- tions with respect to [26] are 70 cm − 1 and 120 GPa. The experimental parameters capture the correct qualitative high- pressure behavior up to 900 GPa, despite the fact that data was available only to 20 and 64 GPa, respectively. This suggests the form for the fit was well-chosen. 

We have presented a fully ab initio pressure calibration based on quantum Monte Carlo simulations, and have intro- duced a method for all-electron simulations of solids to elimi- nate bias from pseudopotentials. This method should be appli- cable to at least first-row solids, allowing increased accur acy in the study of other materials and providing a new benchmark for other methods. The only remaining systematic error in the static contribution to the EOS is from the fixed-node approxi - mation used in DMC. For simple materials such as c-BN this error should be quite small, and tends to cancel between dif- ferent volumes. Thus we believe the EOS is robust enough to be used directly in experiment as a primary pressure calibrant, 

and can be used to cross-calibrate scales based on other ma- terials. Since the accuracy of our methods should not depend on compression, our calibration can be used up to 900 GPa. 

This work was supported under the National Science Foun- dation Grant EAR-0530282. This research used resources of the National Center for Computational Sciences and the Cen- ter for Nanophase Materials Sciences, which are sponsored by the U.S. Department of Energy. This work was partially supported by the National Center for Supercomputing Appli- cations under MCA07S016 and utilized the Abe and Lincoln clusters. We thank Alexander Goncharov, F. Mauri and M. Lazzeri for helpful suggestions and discussions. 

∗ Electronic address: esler@uiuc.edu 

[1] Yingwei Fei et al. , PNAS 104 , 9182 (2007). 

[2] H.K. Mao, J. Xu, and P.M. Bell, J. Geophys. Res. 91 , 4573 (1986). 

[3] Chang-Sheng Zha, Ho-kwang Mao, and Russell J. Hemley, PNAS 97 , 13494 (2000). 

[4] J. R. Asaya et al. , Int. J. of Impact Engineering 23 , 27 (1999). 

[5] J. Remo and M. Furnish, Int. J. of Impact Engineering 35 , 1516 (2008). 

[6] W. B. Holzapfel, High Pressure Research 25 , 87 (2005). 

[7] Alexander F. Goncharov et al , Phys. Rev. B 75 , 224114 (2007). 

[8] W. M. C. Foulkes, L. Mitas, R. J. Needs, and G. Rajagopal, Rev. Mod. Phys. 73 , 33 (2001). 

[9] Ryo Maezono, A. Ma, M.D. Towler, and R.J. Needs, Phys. Rev. Lett. 98 , 025701 (2007). 

[10] J. Kolorenˇc and L. Mitas, Phys. Rev. Lett. 101 , 185502 (2008). 

[11] R. J. Needs, M. D. Towler, N. D. Drummond, and P. L. Rios, J. Phys.: Condensed Matter 22 , 023201 (2010). 

[12] Jeongnim Kim, K. Esler, J. McMinis, B. Clark, J. Gergely, S. Chiesa, K. Delaney, J. Vincent, and D. Ceperley. 

[13] J. K. Dewhurst et al. , The EXCITING FP-LAPW code , http://exciting.sourceforge.net. 

[14] A. J. Williamson et al. , Phys. Rev. B 55 , R4851 (1997). 

[15] S. Chiesa, D.M. Ceperley, R.M. Martin, and M. Holzmann, Phys. Rev. Letts. 97 , 076404 (2006). 

[16] J.R. Trail and R.J. Needs, J. Chem. Phys. 122 , 174109 (2005). 

[17] J.R. Trail and R.J. Needs, J. Chem. Phys. 122 , 014112 (2005). 

[18] M. Burkatzki, C. Filippi, and M. Dolg, J. Chem. Phys. 126 , 234105 (2007). 

[19] Zhigang Wu and R.E. Cohen, Phys. Rev. B 73 , 235116 (2006). 

[20] Opium pseudopotential generation project , http://opium.sourceforge.net. 

[21] S. Baroni et al. , J.Phys.:Condens.Matter 21 , 395502 (2009). 

[22] J. Furthm¨uller, J. Hafner, and G. Kresse, Phys. Rev. B 50 , 15606 (1994). 

[23] P Vinet et al. , J. Phys.: Condensed Matter 1 , 1941 (1989). 

[24] F. Datchi, A. Dewaele, Y. Le Godec, and P. Loubeyre, Phys. Rev. B 75 , 214104 (2007). 

[25] F. Datchi and B. Canny, Phys. Rev. B 69 , 144106 (2004). 

[26] Alexander F. Goncharov, Jonathan C. Crowhurst, J. K. De- whurst, and S. Sharma, Phys. Rev. B 72 , 100104(R) (2005). 

[27] I. V. Alexandrov et al. , High pressure study of diamond, graphite and related materials (Terrepub AGU, Washington- Tokyo, 1992), pp. 409–416. 

