Noncrystallographic symmetry (NCS) averaging following molecular-replacement phasing is generally the major technique used to solve a structure with several molecules in one asymmetric unit, such as a spherical icosahedral viral particle. As an alternative method to NCS averaging, a new approach to optimize or to refine the electron density directly under NCS constraints is proposed. This method has the same effect as the conventional NCS-averaging method but does not include the process of Fourier synthesis to generate the electron density from amplitudes and the corresponding phases. It has great merit for the solution of structures with limited data that are either twinned or incomplete at low resolution. This method was applied to the case of the T = 1 shell-domain subviral particle of Penaeus vannamei nodavirus with data affected by twinning using the REFMAC5 refinement software.

中文翻译：

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通过直接密度优化获得的非晶体对称约束图。

分子置换定相后的非晶体对称性 (NCS) 平均通常是用于求解一个不对称单元中包含多个分子的结构（例如球形二十面体病毒颗粒）的主要技术。作为 NCS 平均的替代方法，提出了一种直接在 NCS 约束下优化或细化电子密度的新方法。该方法与传统的NCS平均方法具有相同的效果，但不包括从振幅和相应相位生成电子密度的傅里叶合成过程。它对于解决数据有限、低分辨率下孪生或不完整的结构具有很大的优点。该方法适用于南美白对虾诺达病毒 T = 1 壳域亚病毒颗粒的情况，其数据受使用 REFMAC5 细化软件的孪生影响。