Exceptional molecules and materials with one or more
extraordinary
properties are both technologically valuable and fundamentally interesting,
because they often involve new physical phenomena or new compositions
that defy expectations. Historically, exceptionality has been achieved
through serendipity, but recently, machine learning (ML) and automated
experimentation have been widely proposed to accelerate target identification
and synthesis planning. In this Perspective, we argue that the data-driven
methods commonly used today are well-suited for optimization but not
for the realization of new exceptional materials or molecules. Finding
such outliers should be possible using ML, but only by shifting away
from using traditional ML approaches that tweak the composition, crystal
structure, or reaction pathway. We highlight case studies of high-T
c oxide superconductors and superhard materials
to demonstrate the challenges of ML-guided discovery and discuss the
limitations of automation for this task. We then provide six recommendations
for the development of ML methods capable of exceptional materials
discovery: (i) Avoid the tyranny of the middle and focus on extrema;
(ii) When data are limited, qualitative predictions that provide direction
are more valuable than interpolative accuracy; (iii) Sample what can
be made and how to make it and defer optimization; (iv) Create room
(and look) for the unexpected while pursuing your goal; (v) Try to
fill-in-the-blanks of input and output space; (vi) Do not confuse
human understanding with model interpretability. We conclude with
a description of how these recommendations can be integrated into
automated discovery workflows, which should enable the discovery of
exceptional molecules and materials.