The static evaluation function returns a score for the side to move from the given position. A score is calculated for both sides and the function returns the score for the side on the move minus the score for the side not on the move.
The factors considered in the evaluation function have been chosen because they are relatively quick to calculate. Very few of the ideas are entirely original; many represent elementary chess knowledge and many have been used in other chess programs. Sources that have been of particular influence are Slate & Atkin (1977), Newborn (1975) and Hyatt et al (1985). Some of the factors have been added to overcome certain weaknesses that the program has shown, others have been left out in the hope that the gain in search speed would outweigh the loss in evaluation quality. The work of Berliner et al (1990) suggests the opposite to the last assumption.
The majority of factors considered by the evaluation function are described below with exact point weightings given where considered appropriate.
Each pawn scores 100 points. A side is penalised seven points for having two or more pawns on the same file (doubled pawns). A two point penalty is inflicted for isolated pawns. Passed pawns are awarded a bonus that relates to the pawn's rank number. If there is a hostile piece in front of a passed pawn, a value, also relating to the pawn's rank number is deducted from the score. Pawns other than those on files one and eight are awarded bonuses for advancement ranging from 3 points for a pawn on the third rank, third file, to 34 points for a pawn on the seventh rank, fifth file.
Each bishop scores 340 points. Each of the squares diagonally adjacent to the bishop's square are considered with a penalty being inflicted for each square that is occupied by a pawn of either colour. A bonus is given for the presence of two bishops.
Each rook scores 500 points. Rooks are awarded a bonus for king tropism that is based on the minimum of the rank and file distances from the enemy king. Rooks on the seventh rank score 20 points. If two friendly rooks share the same file, the side receives a bonus of 15 points. If there are no pawns on the same file as a rook, a bonus of 10 points is given. If there are e nemy pawns on the same file but no friendly pawns, a bonus of three points is given.
Each knight scores 325 points. Knights are awarded bonuses for closeness to the centre of the board ranging from -14 points for a corner square to +7 points for a centre square. Knights are also awarded points for closeness to the enemy king. Unlike rooks, a score is awarded based on the sum of the rank and file distances from the enemy king.
Each queen scores 900 points. Queens are awarded points for closeness to the enemy king. A experimental factor has been added for queen scoring since the game in which position 3.4 occurred; a small bonus is awarded if a queen is on the same diagonal as a friendly bishop.
If the number of enemy pieces and pawns in the friendly king's board quadrant is greater than the number of friendly pieces and pawns in the same quadrant, the side is penalised the difference multiplied by five. When considering enemy presence in the quadrant a queen is counted as three pieces.
If a side has not castled and castling is no longer possible, that side is penalised 15 points. If castling is still possible then a penalty is given if one of the rooks has moved; 12 points for the king's rook, 8 points for the queen's rook.
The evaluation function does not detect checkmate. Evaluation of won, drawn of lost positions is left to a function that is called when a position is found in the search from which there are no available moves. The value of a won position is 10,000 points although the depth at which such a position is discovered is subtracted from this score. This encourages the program to take the shortest sequence of moves to win a game. Similarly, the depth at which lost positions are discovered is added to the value -10,000 to encourage to program to delay the loss for as long as possible in the unsportsmanlike hope that the opponent will make a mistake.